Allow to round taxes on order-level

src/pretix/api/serializers/event.py

@@ -829,6 +829,7 @@ class EventSettingsSerializer(SettingsSerializer):
         'invoice_eu_currencies',
         'invoice_logo_image',
         'invoice_renderer_highlight_order_code',
+        'tax_rounding',
         'cancel_allow_user',
         'cancel_allow_user_until',
         'cancel_allow_user_unpaid_keep',

src/pretix/base/migrations/0285_cartposition_price_includes_rounding_correction_and_more.py

@@ -0,0 +1,76 @@
+# Generated by Django 4.2.17 on 2025-04-20 13:58
+
+from decimal import Decimal
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("pretixbase", "0284_ordersyncresult_ordersyncqueue"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="cartposition",
+            name="price_includes_rounding_correction",
+            field=models.DecimalField(
+                decimal_places=2, default=Decimal("0.00"), max_digits=13
+            ),
+        ),
+        migrations.AddField(
+            model_name="cartposition",
+            name="tax_code",
+            field=models.CharField(max_length=190, null=True),
+        ),
+        migrations.AddField(
+            model_name="cartposition",
+            name="tax_value_includes_rounding_correction",
+            field=models.DecimalField(
+                decimal_places=2, default=Decimal("0.00"), max_digits=13
+            ),
+        ),
+        migrations.AddField(
+            model_name="orderfee",
+            name="tax_value_includes_rounding_correction",
+            field=models.DecimalField(
+                decimal_places=2, default=Decimal("0.00"), max_digits=13
+            ),
+        ),
+        migrations.AddField(
+            model_name="orderfee",
+            name="value_includes_rounding_correction",
+            field=models.DecimalField(
+                decimal_places=2, default=Decimal("0.00"), max_digits=13
+            ),
+        ),
+        migrations.AddField(
+            model_name="orderposition",
+            name="price_includes_rounding_correction",
+            field=models.DecimalField(
+                decimal_places=2, default=Decimal("0.00"), max_digits=13
+            ),
+        ),
+        migrations.AddField(
+            model_name="orderposition",
+            name="tax_value_includes_rounding_correction",
+            field=models.DecimalField(
+                decimal_places=2, default=Decimal("0.00"), max_digits=13
+            ),
+        ),
+        migrations.AddField(
+            model_name="transaction",
+            name="price_includes_rounding_correction",
+            field=models.DecimalField(
+                decimal_places=2, default=Decimal("0.00"), max_digits=13
+            ),
+        ),
+        migrations.AddField(
+            model_name="transaction",
+            name="tax_value_includes_rounding_correction",
+            field=models.DecimalField(
+                decimal_places=2, default=Decimal("0.00"), max_digits=13
+            ),
+        ),
+    ]

src/pretix/base/models/orders.py

@@ -1259,7 +1259,8 @@ class Order(LockModel, LoggedModel):
         keys = set(target_transaction_count.keys()) | set(current_transaction_count.keys())
         create = []
         for k in keys:
-            positionid, itemid, variationid, subeventid, price, taxrate, taxruleid, taxvalue, feetype, internaltype, taxcode = k
+            (positionid, itemid, variationid, subeventid, price, price_includes_rounding_correction, taxrate,
+             taxruleid, taxvalue, taxvalue_includes_rounding_correction, feetype, internaltype, taxcode) = k
             d = target_transaction_count[k] - current_transaction_count[k]
             if d:
                 create.append(Transaction(
@@ -1272,9 +1273,11 @@ class Order(LockModel, LoggedModel):
                     variation_id=variationid,
                     subevent_id=subeventid,
                     price=price,
+                    price_includes_rounding_correction=price_includes_rounding_correction,
                     tax_rate=taxrate,
                     tax_rule_id=taxruleid,
                     tax_value=taxvalue,
+                    tax_value_includes_rounding_correction=taxvalue_includes_rounding_correction,
                     tax_code=taxcode,
                     fee_type=feetype,
                     internal_type=internaltype,
@@ -1499,6 +1502,9 @@ class AbstractPosition(models.Model):
         decimal_places=2, max_digits=13,
         verbose_name=_("Price")
     )
+    price_includes_rounding_correction = models.DecimalField(
+        max_digits=13, decimal_places=2, default=Decimal("0.00")
+    )
     attendee_name_cached = models.CharField(
         max_length=255,
         verbose_name=_("Attendee name"),
@@ -2301,6 +2307,9 @@ class OrderFee(models.Model):
         decimal_places=2, max_digits=13,
         verbose_name=_("Value")
     )
+    value_includes_rounding_correction = models.DecimalField(
+        max_digits=13, decimal_places=2, default=Decimal("0.00")
+    )
     order = models.ForeignKey(
         Order,
         verbose_name=_("Order"),
@@ -2329,6 +2338,9 @@ class OrderFee(models.Model):
         max_digits=13, decimal_places=2,
         verbose_name=_('Tax value')
     )
+    tax_value_includes_rounding_correction = models.DecimalField(
+        max_digits=13, decimal_places=2, default=Decimal("0.00")
+    )
     canceled = models.BooleanField(default=False)
 
     all = ScopedManager(organizer='order__event__organizer')
@@ -2377,17 +2389,23 @@ class OrderFee(models.Model):
             self.fee_type, self.value
         )
 
-    def _calculate_tax(self, tax_rule=None, invoice_address=None):
+    def _calculate_tax(self, tax_rule=None, invoice_address=None, event=None):
         if tax_rule:
             self.tax_rule = tax_rule
 
-        try:
-            ia = invoice_address or self.order.invoice_address
-        except InvoiceAddress.DoesNotExist:
+        if invoice_address:
+            ia = invoice_address
+        elif hasattr(self, "order"):
+            try:
+                ia = self.order.invoice_address
+            except InvoiceAddress.DoesNotExist:
+                ia = None
+        else:
             ia = None
 
-        if not self.tax_rule and self.fee_type == "payment" and self.order.event.settings.tax_rule_payment == "default":
-            self.tax_rule = self.order.event.cached_default_tax_rule
+        event = event or self.order.event
+        if not self.tax_rule and self.fee_type == "payment" and event.settings.tax_rule_payment == "default":
+            self.tax_rule = event.cached_default_tax_rule
 
         if self.tax_rule:
             tax = self.tax_rule.tax(self.value, base_price_is='gross', invoice_address=ia, force_fixed_gross_price=True)
@@ -2422,6 +2440,24 @@ class OrderFee(models.Model):
         self.order.touch()
         super().delete(**kwargs)
 
+    # For historical reasons, OrderFee has "value", but OrderPosition has "price". These properties
+    # help using them the same way.
+    @property
+    def price(self):
+        return self.value
+
+    @price.setter
+    def price(self, value):
+        self.value = value
+
+    @property
+    def price_includes_rounding_correction(self):
+        return self.value_includes_rounding_correction
+
+    @price_includes_rounding_correction.setter
+    def price_includes_rounding_correction(self, value):
+        self.value_includes_rounding_correction = value
+
 
 class OrderPosition(AbstractPosition):
     """
@@ -2501,6 +2537,9 @@ class OrderPosition(AbstractPosition):
         max_digits=13, decimal_places=2,
         verbose_name=_('Tax value')
     )
+    tax_value_includes_rounding_correction = models.DecimalField(
+        max_digits=13, decimal_places=2, default=Decimal("0.00"),
+    )
 
     secret = models.CharField(max_length=255, null=False, blank=False, db_index=True)
     web_secret = models.CharField(max_length=32, default=generate_secret, db_index=True)
@@ -2673,7 +2712,14 @@ class OrderPosition(AbstractPosition):
                         setattr(op, f.name, cp_mapping[cartpos.addon_to_id])
                 else:
                     setattr(op, f.name, getattr(cartpos, f.name))
-            op._calculate_tax()
+
+            op.tax_value = cartpos.tax_value
+            op.tax_value_includes_rounding_correction = cartpos.tax_value_includes_rounding_correction
+            op.tax_rate = cartpos.tax_rate
+            op.tax_code = cartpos.tax_code
+            op.tax_rule = cartpos.item.tax_rule
+            # todo: is removing this safe? op._calculate_tax()
+
             if cartpos.voucher:
                 op.voucher_budget_use = cartpos.listed_price - cartpos.price_after_voucher
 
@@ -3006,6 +3052,9 @@ class Transaction(models.Model):
         decimal_places=2, max_digits=13,
         verbose_name=_("Price")
     )
+    price_includes_rounding_correction = models.DecimalField(
+        max_digits=13, decimal_places=2, default=Decimal("0.00")
+    )
     tax_rate = models.DecimalField(
         max_digits=7, decimal_places=2,
         verbose_name=_('Tax rate')
@@ -3023,6 +3072,9 @@ class Transaction(models.Model):
         max_digits=13, decimal_places=2,
         verbose_name=_('Tax value')
     )
+    tax_value_includes_rounding_correction = models.DecimalField(
+        max_digits=13, decimal_places=2, default=Decimal("0.00")
+    )
     fee_type = models.CharField(
         max_length=100, choices=OrderFee.FEE_TYPES, null=True, blank=True
     )
@@ -3052,14 +3104,19 @@ class Transaction(models.Model):
     @staticmethod
     def key(obj):
         if isinstance(obj, Transaction):
-            return (obj.positionid, obj.item_id, obj.variation_id, obj.subevent_id, obj.price, obj.tax_rate,
-                    obj.tax_rule_id, obj.tax_value, obj.fee_type, obj.internal_type, obj.tax_code)
+            return (obj.positionid, obj.item_id, obj.variation_id, obj.subevent_id, obj.price,
+                    obj.price_includes_rounding_correction, obj.tax_rate, obj.tax_rule_id,
+                    obj.tax_value, obj.tax_value_includes_rounding_correction, obj.fee_type,
+                    obj.internal_type, obj.tax_code)
         elif isinstance(obj, OrderPosition):
-            return (obj.positionid, obj.item_id, obj.variation_id, obj.subevent_id, obj.price, obj.tax_rate,
-                    obj.tax_rule_id, obj.tax_value, None, None, obj.tax_code)
+            return (obj.positionid, obj.item_id, obj.variation_id, obj.subevent_id, obj.price,
+                    obj.price_includes_rounding_correction, obj.tax_rate, obj.tax_rule_id,
+                    obj.tax_value, obj.tax_value_includes_rounding_correction, None,
+                    None, obj.tax_code)
         elif isinstance(obj, OrderFee):
-            return (None, None, None, None, obj.value, obj.tax_rate,
-                    obj.tax_rule_id, obj.tax_value, obj.fee_type, obj.internal_type, obj.tax_code)
+            return (None, None, None, None, obj.value, obj.value_includes_rounding_correction,
+                    obj.tax_rate, obj.tax_rule_id, obj.tax_value, obj.tax_value_includes_rounding_correction,
+                    obj.fee_type, obj.internal_type, obj.tax_code)
         raise ValueError('invalid state')  # noqa
 
     @property
@@ -3070,6 +3127,14 @@ class Transaction(models.Model):
     def full_tax_value(self):
         return self.tax_value * self.count
 
+    @property
+    def full_price_includes_rounding_correction(self):
+        return self.price_includes_rounding_correction * self.count
+
+    @property
+    def full_tax_value_includes_rounding_correction(self):
+        return self.tax_value_includes_rounding_correction * self.count
+
 
 class CartPosition(AbstractPosition):
     """
@@ -3110,6 +3175,13 @@ class CartPosition(AbstractPosition):
         max_digits=7, decimal_places=2, default=Decimal('0.00'),
         verbose_name=_('Tax rate')
     )
+    tax_code = models.CharField(
+        max_length=190,
+        null=True, blank=True,
+    )
+    tax_value_includes_rounding_correction = models.DecimalField(
+        max_digits=13, decimal_places=2, default=Decimal("0.00")
+    )
     listed_price = models.DecimalField(
         decimal_places=2, max_digits=13, null=True,
     )
@@ -3150,9 +3222,15 @@ class CartPosition(AbstractPosition):
 
     @property
     def tax_value(self):
-        net = round_decimal(self.price - (self.price * (1 - 100 / (100 + self.tax_rate))),
+        price = self.price - self.price_includes_rounding_correction
+        net = round_decimal(price - (price * (1 - 100 / (100 + self.tax_rate))),
                             self.event.currency)
-        return self.price - net
+        return self.price - self.price_includes_rounding_correction - net + self.tax_value_includes_rounding_correction
+
+    @tax_value.setter
+    def tax_value(self, value):
+        # ignore, tax value is always computed on the fly
+        pass
 
     @cached_property
     def sort_key(self):

src/pretix/base/services/orders.py

@@ -93,7 +93,7 @@ from pretix.base.services.memberships import (
     create_membership, validate_memberships_in_order,
 )
 from pretix.base.services.pricing import (
-    apply_discounts, get_listed_price, get_price,
+    apply_discounts, apply_rounding, get_listed_price, get_price,
 )
 from pretix.base.services.quotas import QuotaAvailability
 from pretix.base.services.tasks import ProfiledEventTask, ProfiledTask
@@ -939,7 +939,8 @@ def _check_positions(event: Event, now_dt: datetime, time_machine_now_dt: dateti
 def _get_fees(positions: List[CartPosition], payment_requests: List[dict], address: InvoiceAddress,
               meta_info: dict, event: Event, require_approval=False):
     fees = []
-    total = sum([c.price for c in positions])
+    # Pre-rounding, pre-fee total is used for fee calculation
+    total = sum([c.price - c.price_includes_rounding_correction for c in positions])
 
     gift_cards = []  # for backwards compatibility
     for p in payment_requests:
@@ -950,40 +951,53 @@ def _get_fees(positions: List[CartPosition], payment_requests: List[dict], addre
                                                  meta_info=meta_info, positions=positions, gift_cards=gift_cards):
         if resp:
             fees += resp
-    total += sum(f.value for f in fees)
 
-    total_remaining = total
+    for fee in fees:
+        fee._calculate_tax(invoice_address=address, event=event)
+        if fee.tax_rule and not fee.tax_rule.pk:
+            fee.tax_rule = None  # TODO: deprecate
+
+    # Apply rounding to get final total in case no payment fees will be added
+    apply_rounding(event.settings.tax_rounding, event.currency, [*positions, *fees])
+    total = sum([c.price for c in positions]) + sum([f.value for f in fees])
+
+    payments_assigned = Decimal("0.00")
     for p in payment_requests:
         # This algorithm of treating min/max values and fees needs to stay in sync between the following
         # places in the code base:
         # - pretix.base.services.cart.get_fees
         # - pretix.base.services.orders._get_fees
         # - pretix.presale.views.CartMixin.current_selected_payments
-        if p.get('min_value') and total_remaining < Decimal(p['min_value']):
+        if p.get('min_value') and total - payments_assigned < Decimal(p['min_value']):
             p['payment_amount'] = Decimal('0.00')
             continue
 
-        to_pay = total_remaining
+        to_pay = max(total - payments_assigned, Decimal("0.00"))
         if p.get('max_value') and to_pay > Decimal(p['max_value']):
             to_pay = min(to_pay, Decimal(p['max_value']))
 
         payment_fee = p['pprov'].calculate_fee(to_pay)
-        total_remaining += payment_fee
-        to_pay += payment_fee
-
-        if p.get('max_value') and to_pay > Decimal(p['max_value']):
-            to_pay = min(to_pay, Decimal(p['max_value']))
-
-        total_remaining -= to_pay
-
-        p['payment_amount'] = to_pay
         if payment_fee:
             pf = OrderFee(fee_type=OrderFee.FEE_TYPE_PAYMENT, value=payment_fee,
                           internal_type=p['pprov'].identifier)
+            pf._calculate_tax(invoice_address=address, event=event)
             fees.append(pf)
             p['fee'] = pf
 
-    if total_remaining != Decimal('0.00') and not require_approval:
+            # Re-apply rounding as grand total has changed
+            apply_rounding(event.settings.tax_rounding, event.currency, [*positions, *fees])
+            total = sum([c.price for c in positions]) + sum([f.value for f in fees])
+
+            # Re-calculate to_pay as grand total has changed
+            to_pay = max(total - payments_assigned, Decimal("0.00"))
+
+            if p.get('max_value') and to_pay > Decimal(p['max_value']):
+                to_pay = min(to_pay, Decimal(p['max_value']))
+
+        payments_assigned += to_pay
+        p['payment_amount'] = to_pay
+
+    if total != payments_assigned and not require_approval:
         raise OrderError(_("The selected payment methods do not cover the total balance."))
 
     return fees
@@ -1001,10 +1015,13 @@ def _create_order(event: Event, *, email: str, positions: List[CartPosition], no
         raise OrderError(e.message)
 
     require_approval = any(p.requires_approval(invoice_address=address) for p in positions)
+
+    # Final calculation of fees, also performs final rounding
     try:
         fees = _get_fees(positions, payment_requests, address, meta_info, event, require_approval=require_approval)
     except TaxRule.SaleNotAllowed:
         raise OrderError(error_messages['country_blocked'])
+
     total = pending_sum = sum([c.price for c in positions]) + sum([c.value for c in fees])
 
     order = Order(
@@ -1036,12 +1053,6 @@ def _create_order(event: Event, *, email: str, positions: List[CartPosition], no
 
     for fee in fees:
         fee.order = order
-        try:
-            fee._calculate_tax()
-        except TaxRule.SaleNotAllowed:
-            raise OrderError(error_messages['country_blocked'])
-        if fee.tax_rule and not fee.tax_rule.pk:
-            fee.tax_rule = None  # TODO: deprecate
         fee.save()
 
     # Safety check: Is the amount we're now going to charge the same amount the user has been shown when they
@@ -2696,9 +2707,12 @@ class OrderChangeManager:
         ).aggregate(s=Sum('amount'))['s'] or Decimal('0.00')
         return payment_sum - refund_sum
 
-    def _recalculate_total_and_payment_fee(self):
-        total = sum([p.price for p in self.order.positions.all()]) + sum([f.value for f in self.order.fees.all()])
+    def _recalculate_rounding_total_and_payment_fee(self):
+        positions = list(self.order.positions.all())
+        fees = list(self.order.fees.all())
+        total = sum([p.price for p in positions]) + sum([f.value for f in fees])
         payment_fee = Decimal('0.00')
+        fee_changed = False
         if self.open_payment:
             current_fee = Decimal('0.00')
             fee = None
@@ -2726,13 +2740,31 @@ class OrderChangeManager:
                 fee.value = payment_fee
                 fee._calculate_tax()
                 fee.save()
+                fee_changed = True
                 if not self.open_payment.fee:
                     self.open_payment.fee = fee
                     self.open_payment.save(update_fields=['fee'])
             elif fee and not fee.canceled:
                 fee.delete()
+                fee_changed = True
 
-        self.order.total = total + payment_fee
+        if fee_changed:
+            fees = list(self.order.fees.all())
+
+        print("round", positions, fees)
+        changed = apply_rounding(self.order.event.settings.tax_rounding, self.order.event.currency, [*positions, *fees])
+        for l in changed:
+            if isinstance(l, OrderPosition):
+                l.save(update_fields=[
+                    "price", "price_includes_rounding_correction", "tax_value", "tax_value_includes_rounding_correction"
+                ])
+            elif isinstance(l, OrderFee):
+                l.save(update_fields=[
+                    "value", "value_includes_rounding_correction", "tax_value", "tax_value_includes_rounding_correction"
+                ])
+        total = sum([p.price for p in positions]) + sum([f.value for f in fees])
+
+        self.order.total = total
         self.order.save()
 
     def _check_order_size(self):
@@ -2913,7 +2945,7 @@ class OrderChangeManager:
                 self._perform_operations()
             except TaxRule.SaleNotAllowed:
                 raise OrderError(self.error_messages['tax_rule_country_blocked'])
-            self._recalculate_total_and_payment_fee()
+            self._recalculate_rounding_total_and_payment_fee()
             self._check_paid_price_change()
             self._check_paid_to_free()
             if self.order.status in (Order.STATUS_PENDING, Order.STATUS_PAID):

src/pretix/base/services/pricing.py

@@ -23,15 +23,17 @@ import re
 from collections import defaultdict
 from datetime import datetime
 from decimal import Decimal
-from typing import List, Optional, Tuple, Union
+from itertools import groupby
+from typing import List, Literal, Optional, Tuple, Union
 
 from django import forms
+from django.conf import settings
 from django.db.models import Q
 
 from pretix.base.decimal import round_decimal
 from pretix.base.models import (
-    AbstractPosition, InvoiceAddress, Item, ItemAddOn, ItemVariation,
-    SalesChannel, Voucher,
+    AbstractPosition, CartPosition, InvoiceAddress, Item, ItemAddOn,
+    ItemVariation, OrderFee, OrderPosition, SalesChannel, Voucher,
 )
 from pretix.base.models.discount import Discount, PositionInfo
 from pretix.base.models.event import Event, SubEvent
@@ -136,7 +138,8 @@ def get_listed_price(item: Item, variation: ItemVariation = None, subevent: SubE
 
 
 def get_line_price(price_after_voucher: Decimal, custom_price_input: Decimal, custom_price_input_is_net: bool,
-                   tax_rule: TaxRule, invoice_address: InvoiceAddress, bundled_sum: Decimal, is_bundled=False) -> TaxedPrice:
+                   tax_rule: TaxRule, invoice_address: InvoiceAddress, bundled_sum: Decimal,
+                   is_bundled=False) -> TaxedPrice:
     if not tax_rule:
         tax_rule = TaxRule(
             name='',
@@ -152,7 +155,8 @@ def get_line_price(price_after_voucher: Decimal, custom_price_input: Decimal, cu
                                  override_tax_code=price.code, invoice_address=invoice_address,
                                  subtract_from_gross=bundled_sum)
         else:
-            price = tax_rule.tax(max(custom_price_input, price.gross), base_price_is='gross', override_tax_rate=price.rate,
+            price = tax_rule.tax(max(custom_price_input, price.gross), base_price_is='gross',
+                                 override_tax_rate=price.rate,
                                  override_tax_code=price.code, invoice_address=invoice_address,
                                  subtract_from_gross=bundled_sum)
     else:
@@ -164,7 +168,7 @@ def get_line_price(price_after_voucher: Decimal, custom_price_input: Decimal, cu
 
 def apply_discounts(event: Event, sales_channel: Union[str, SalesChannel],
                     positions: List[Tuple[int, Optional[int], Optional[datetime], Decimal, bool, bool, Decimal]],
-                    collect_potential_discounts: Optional[defaultdict]=None) -> List[Tuple[Decimal, Optional[Discount]]]:
+                    collect_potential_discounts: Optional[defaultdict] = None) -> List[Tuple[Decimal, Optional[Discount]]]:
     """
     Applies any dynamic discounts to a cart
 
@@ -203,3 +207,102 @@ def apply_discounts(event: Event, sales_channel: Union[str, SalesChannel],
         new_prices.update(result)
 
     return [new_prices.get(idx, (p[3], None)) for idx, p in enumerate(positions)]
+
+
+def apply_rounding(rounding_mode: Literal["line", "sum_by_gross", "sum_by_net"], currency: str,
+                   lines: List[Union[OrderPosition, CartPosition, OrderFee]]) -> list:
+    """
+    Given a a list of order positions / cart positions / order fees (may be mixed), applies the given rounding mode
+    and mutates the ``price``, ``price_includes_rounding_correction``, ``tax_value``, and
+    ``tax_value_includes_rounding_correction`` attributes.
+
+    When rounding mode is set to ``"line"``, the tax will be computed and rounded individually for every line.
+
+    When rounding mode is set to ``"sum_by_gross"``, the tax values of the individual lines will be adjusted such that
+    the per-taxrate/taxcode subtotal is rounded correctly. The gross prices will stay constant.
+
+    When rounding mode is set to ``"sum_by_net"``, the gross prices and tax values of the individual lines will be
+    adjusted such that the per-taxrate/taxcode subtotal is rounded correctly. The net prices will stay constant.
+
+    :param rounding_mode: One of ``"line"``, ``"sum_by_gross"``, or ``"sum_by_net"``.
+    :param currency: Currency that will be used to determine rounding precision
+    :param lines: List of order/cart contents
+    :return: Collection of ``lines`` members that have been changed and may need to be persisted to the database.
+    """
+
+    def _key(line):
+        return (line.tax_rate, line.tax_code)
+
+    places = settings.CURRENCY_PLACES.get(currency, 2)
+    minimum_unit = Decimal('1') / 10 ** places
+    changed = []
+
+    if rounding_mode == "sum_by_net":
+        for (tax_rate, tax_code), lines in groupby(sorted(lines, key=_key), key=_key):
+            lines = list(sorted(lines, key=lambda l: -(l.price - l.price_includes_rounding_correction)))
+            net_total = sum(
+                l.price - l.price_includes_rounding_correction - l.tax_value + l.tax_value_includes_rounding_correction
+                for l in lines
+            )
+            gross_total = sum(l.price - l.price_includes_rounding_correction for l in lines)
+            target_gross_total = round_decimal((net_total * (1 + tax_rate / 100)), currency)
+
+            diff = target_gross_total - gross_total
+            diff_sgn = -1 if diff < 0 else 1
+            for l in lines:
+                if diff:
+                    apply_diff = diff_sgn * minimum_unit
+                    l.price = l.price - l.price_includes_rounding_correction + apply_diff
+                    l.price_includes_rounding_correction = diff_sgn * minimum_unit
+                    l.tax_value = l.tax_value - l.tax_value_includes_rounding_correction + apply_diff
+                    l.tax_value_includes_rounding_correction = apply_diff
+                    diff -= apply_diff
+                    changed.append(l)
+                elif l.price_includes_rounding_correction or l.tax_value_includes_rounding_correction:
+                    l.price = l.price - l.price_includes_rounding_correction
+                    l.price_includes_rounding_correction = Decimal("0.00")
+                    l.tax_value = l.tax_value - l.tax_value_includes_rounding_correction
+                    l.tax_value_includes_rounding_correction = Decimal("0.00")
+                    changed.append(l)
+
+    elif rounding_mode == "sum_by_gross":
+        for (tax_rate, tax_code), lines in groupby(sorted(lines, key=_key), key=_key):
+            lines = list(sorted(lines, key=lambda l: -(l.price - l.price_includes_rounding_correction)))
+            net_total = sum(
+                l.price - l.price_includes_rounding_correction - l.tax_value + l.tax_value_includes_rounding_correction
+                for l in lines
+            )
+            gross_total = sum(l.price - l.price_includes_rounding_correction for l in lines)
+            target_net_total = round_decimal(gross_total - (gross_total * (1 - 100 / (100 + tax_rate))), currency)
+
+            diff = target_net_total - net_total
+            diff_sgn = -1 if diff < 0 else 1
+            for l in lines:
+                if diff:
+                    apply_diff = diff_sgn * minimum_unit
+                    l.price = l.price - l.price_includes_rounding_correction
+                    l.price_includes_rounding_correction = Decimal("0.00")
+                    l.tax_value = l.tax_value - l.tax_value_includes_rounding_correction - apply_diff
+                    l.tax_value_includes_rounding_correction = -apply_diff
+                    diff -= apply_diff
+                    changed.append(l)
+                elif l.price_includes_rounding_correction or l.tax_value_includes_rounding_correction:
+                    l.price = l.price - l.price_includes_rounding_correction
+                    l.price_includes_rounding_correction = Decimal("0.00")
+                    l.tax_value = l.tax_value - l.tax_value_includes_rounding_correction
+                    l.tax_value_includes_rounding_correction = Decimal("0.00")
+                    changed.append(l)
+
+    elif rounding_mode == "line":
+        for l in lines:
+            if l.price_includes_rounding_correction or l.tax_value_includes_rounding_correction:
+                l.price = l.price - l.price_includes_rounding_correction
+                l.price_includes_rounding_correction = Decimal("0.00")
+                l.tax_value = l.tax_value - l.tax_value_includes_rounding_correction
+                l.tax_value_includes_rounding_correction = Decimal("0.00")
+                changed.append(l)
+
+    else:
+        raise ValueError("Unknown rounding_mode")
+
+    return changed

src/pretix/base/settings.py

@@ -465,6 +465,28 @@ DEFAULTS = {
             widget=forms.CheckboxInput(attrs={'data-checkbox-dependency': '#id_settings-order_phone_asked'}),
         )
     },
+    'tax_rounding': {
+        'default': 'line',
+        'type': str,
+        'form_class': forms.ChoiceField,
+        'serializer_class': serializers.ChoiceField,
+        'form_kwargs': dict(
+            label=_("Rounding of taxes"),
+            widget=forms.RadioSelect,
+            choices=(
+                ('line', _('Rounding every line individually')),
+                ('sum_by_net', _('Rounding by order total, keeping net prices stable')),
+                ('sum_by_gross', _('Rounding by order total, keeping gross prices stable')),
+            ),
+        ),
+        'serializer_kwargs': dict(
+            choices=(
+                ('line', _('Rounding every line individually')),
+                ('sum_by_net', _('Rounding by order total, keeping net prices stable')),
+                ('sum_by_gross', _('Rounding by order total, keeping gross prices stable')),
+            ),
+        ),
+    },
     'invoice_address_asked': {
         'default': 'True',
         'type': bool,

src/pretix/control/forms/event.py

@@ -881,6 +881,7 @@ class InvoiceSettingsForm(EventSettingsValidationMixin, SettingsForm):
         'invoice_logo_image',
         'invoice_renderer_highlight_order_code',
         'invoice_renderer_font',
+        'tax_rounding',
     ]
 
     invoice_generate_sales_channels = forms.MultipleChoiceField(

src/pretix/control/templates/pretixcontrol/event/invoicing.html

@@ -21,6 +21,7 @@
                 {% bootstrap_field form.invoice_numbers_prefix layout="control" %}
                 {% bootstrap_field form.invoice_numbers_prefix_cancellations layout="control" %}
                 {% bootstrap_field form.invoice_numbers_counter_length layout="control" %}
+                {% bootstrap_field form.tax_rounding layout="control" %}
             </fieldset>
             <fieldset>
                 <legend>{% trans "Address form" %}</legend>

src/pretix/control/templates/pretixcontrol/order/index.html

@@ -641,6 +641,16 @@
                                         </small>
                                     {% endif %}
                                 {% endif %}
+                                {% if django_settings.DEBUG %}
+                                    <br/>
+                                    <small>
+                                        price = {{ line.price|floatformat:2 }}<br>
+                                        rounding_correction = {{ line.price_includes_rounding_correction|floatformat:2 }}<br>
+                                        tax_value = {{ line.tax_value|floatformat:2 }}<br>
+                                        tax_value_rounding_correction = {{ line.tax_value_includes_rounding_correction|floatformat:2 }}<br>
+                                        voucher_budget_use = {{ line.voucher_budget_use|floatformat:2 }}
+                                    </small>
+                                {% endif %}
                             </div>
                             <div class="clearfix"></div>
                         </div>
@@ -681,6 +691,16 @@
                                         </small>
                                     {% endif %}
                                 {% endif %}
+                                {% if django_settings.DEBUG %}
+                                    <br/>
+                                    <small>
+                                        price = {{ fee.value|floatformat:2 }}<br>
+                                        rounding_correction = {{ fee.value_includes_rounding_correction|floatformat:2 }}<br>
+                                        tax_value = {{ fee.tax_value|floatformat:2 }}<br>
+                                        tax_value_rounding_correction = {{ fee.tax_value_includes_rounding_correction|floatformat:2 }}<br>
+                                        voucher_budget_use = {{ fee.voucher_budget_use|floatformat:2 }}
+                                    </small>
+                                {% endif %}
                             </div>
                             <div class="clearfix"></div>
                         </div>

src/pretix/control/templates/pretixcontrol/order/transactions.html

@@ -56,8 +56,26 @@
                 <td>{{ t.get_tax_code_display }}</td>
                 <td class="text-right flip">{{ t.count }} &times;</td>
                 <td class="text-right flip">{{ t.price|money:request.event.currency }}</td>
-                <td class="text-right flip">{{ t.full_tax_value|money:request.event.currency }}</td>
-                <td class="text-right flip">{{ t.full_price|money:request.event.currency }}</td>
+                <td class="text-right flip">
+                    {{ t.full_tax_value|money:request.event.currency }}
+                    {% if t.full_price_includes_rounding_correction %}
+                        <br><small>
+                            {% blocktrans trimmed with amount=t.full_tax_value_includes_rounding_correction|money:request.event.currency %}
+                                incl. {{ amount }} rounding correction
+                            {% endblocktrans %}
+                        </small>
+                    {% endif %}
+                </td>
+                <td class="text-right flip">
+                    {{ t.full_price|money:request.event.currency }}
+                    {% if t.full_price_includes_rounding_correction %}
+                        <br><small>
+                        {% blocktrans trimmed with amount=t.full_price_includes_rounding_correction|money:request.event.currency %}
+                            incl. {{ amount }} rounding correction
+                        {% endblocktrans %}
+                    </small>
+                    {% endif %}
+                </td>
             </tr>
         {% endfor %}
         </tbody>

src/pretix/control/views/orders.py

@@ -649,7 +649,9 @@ class OrderTransactions(OrderView):
         ctx['sums'] = self.order.transactions.aggregate(
             sum_count=Sum('count'),
             full_price=Sum(F('count') * F('price')),
+            full_price_includes_rounding_correction=Sum(F('count') * F('price_includes_rounding_correction')),
             full_tax_value=Sum(F('count') * F('tax_value')),
+            full_tax_value_includes_rounding_correction=Sum(F('count') * F('tax_value_includes_rounding_correction')),
         )
         return ctx
 

src/pretix/presale/views/__init__.py

@@ -54,6 +54,7 @@ from pretix.base.models import (
     QuestionAnswer, QuestionOption, TaxRule,
 )
 from pretix.base.services.cart import get_fees
+from pretix.base.services.pricing import apply_rounding
 from pretix.base.templatetags.money import money_filter
 from pretix.helpers.cookies import set_cookie_without_samesite
 from pretix.multidomain.urlreverse import eventreverse
@@ -147,6 +148,34 @@ class CartMixin:
                             'question': value.label
                         })
 
+        total = sum(p.price for p in lcp)
+
+        if order:
+            fees = order.fees.all()
+        elif lcp:
+            try:
+                fees = get_fees(
+                    self.request.event, self.request, total, self.invoice_address,
+                    payments if payments is not None else self.cart_session.get('payments', []),
+                    cartpos
+                )
+            except TaxRule.SaleNotAllowed:
+                # ignore for now, will fail on order creation
+                fees = []
+        else:
+            fees = []
+
+        if not order:
+            apply_rounding(self.request.event.settings.tax_rounding, self.request.event.currency, [*lcp, *fees])
+            print([(p.price, p.tax_value, ) for p in lcp])
+            total = sum(p.price for p in lcp)
+
+        net_total = sum(p.price - p.tax_value for p in lcp)
+        tax_total = sum(p.tax_value for p in lcp)
+        total += sum([f.value for f in fees])
+        net_total += sum([f.net_value for f in fees])
+        tax_total += sum([f.tax_value for f in fees])
+
         # Group items of the same variation
         # We do this by list manipulations instead of a GROUP BY query, as
         # Django is unable to join related models in a .values() query
@@ -177,7 +206,7 @@ class CartMixin:
                     pos.subevent_id,
                     pos.item_id,
                     pos.variation_id,
-                    pos.price,
+                    pos.net_price if self.request.event.settings.display_net_prices else pos.price,
                     (pos.voucher_id or 0),
                     (pos.seat_id or 0),
                     pos.valid_from,
@@ -204,29 +233,6 @@ class CartMixin:
                 group.additional_answers = pos_additional_fields.get(group.pk)
             positions.append(group)
 
-        total = sum(p.total for p in positions)
-        net_total = sum(p.net_total for p in positions)
-        tax_total = sum(p.total - p.net_total for p in positions)
-
-        if order:
-            fees = order.fees.all()
-        elif positions:
-            try:
-                fees = get_fees(
-                    self.request.event, self.request, total, self.invoice_address,
-                    payments if payments is not None else self.cart_session.get('payments', []),
-                    cartpos
-                )
-            except TaxRule.SaleNotAllowed:
-                # ignore for now, will fail on order creation
-                fees = []
-        else:
-            fees = []
-
-        total += sum([f.value for f in fees])
-        net_total += sum([f.net_value for f in fees])
-        tax_total += sum([f.tax_value for f in fees])
-
         try:
             first_expiry = min(p.expires for p in positions) if positions else now()
             max_expiry_extend = min((p.max_extend for p in positions if p.max_extend), default=None)

src/tests/base/test_orders.py

@@ -2270,7 +2270,7 @@ class OrderChangeManagerTests(TestCase):
     def test_recalculate_country_rate(self):
         prov = self.ocm._get_payment_provider()
         prov.settings.set('_fee_abs', Decimal('0.30'))
-        self.ocm._recalculate_total_and_payment_fee()
+        self.ocm._recalculate_rounding_total_and_payment_fee()
 
         assert self.order.total == Decimal('46.30')
         fee = self.order.fees.get(fee_type=OrderFee.FEE_TYPE_PAYMENT)
@@ -2302,7 +2302,7 @@ class OrderChangeManagerTests(TestCase):
     def test_recalculate_country_rate_keep_gross(self):
         prov = self.ocm._get_payment_provider()
         prov.settings.set('_fee_abs', Decimal('0.30'))
-        self.ocm._recalculate_total_and_payment_fee()
+        self.ocm._recalculate_rounding_total_and_payment_fee()
 
         assert self.order.total == Decimal('46.30')
         fee = self.order.fees.get(fee_type=OrderFee.FEE_TYPE_PAYMENT)
@@ -2332,7 +2332,7 @@ class OrderChangeManagerTests(TestCase):
     def test_recalculate_reverse_charge(self):
         prov = self.ocm._get_payment_provider()
         prov.settings.set('_fee_abs', Decimal('0.30'))
-        self.ocm._recalculate_total_and_payment_fee()
+        self.ocm._recalculate_rounding_total_and_payment_fee()
 
         assert self.order.total == Decimal('46.30')
         fee = self.order.fees.get(fee_type=OrderFee.FEE_TYPE_PAYMENT)

src/tests/base/test_pricing_rounding.py

@@ -0,0 +1,118 @@
+#
+# This file is part of pretix (Community Edition).
+#
+# Copyright (C) 2014-2020 Raphael Michel and contributors
+# Copyright (C) 2020-2021 rami.io GmbH and contributors
+#
+# This program is free software: you can redistribute it and/or modify it under the terms of the GNU Affero General
+# Public License as published by the Free Software Foundation in version 3 of the License.
+#
+# ADDITIONAL TERMS APPLY: Pursuant to Section 7 of the GNU Affero General Public License, additional terms are
+# applicable granting you additional permissions and placing additional restrictions on your usage of this software.
+# Please refer to the pretix LICENSE file to obtain the full terms applicable to this work. If you did not receive
+# this file, see <https://pretix.eu/about/en/license>.
+#
+# This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied
+# warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Affero General Public License for more
+# details.
+#
+# You should have received a copy of the GNU Affero General Public License along with this program.  If not, see
+# <https://www.gnu.org/licenses/>.
+#
+from decimal import Decimal
+
+import pytest
+
+from pretix.base.models import OrderPosition
+from pretix.base.services.pricing import apply_rounding
+
+
+@pytest.fixture
+def sample_lines():
+    lines = [OrderPosition(
+        price=Decimal("100.00"),
+        tax_value=Decimal("15.97"),
+        tax_rate=Decimal("19.00"),
+        tax_code="S",
+    ) for _ in range(5)]
+    return lines
+
+
+def _validate_sample_lines(sample_lines, rounding_mode):
+    apply_rounding(rounding_mode, "EUR", sample_lines)
+    if rounding_mode == "line":
+        for l in sample_lines:
+            assert l.price == Decimal("100.00")
+            assert l.tax_value == Decimal("15.97")
+            assert l.tax_rate == Decimal("19.00")
+        assert sum(l.price for l in sample_lines) == Decimal("500.00")
+        assert sum(l.tax_value for l in sample_lines) == Decimal("79.85")
+    elif rounding_mode == "sum_by_net":
+        for l in sample_lines:
+            # gross price may vary
+            assert l.price - l.tax_value == Decimal("84.03")
+            assert l.tax_rate == Decimal("19.00")
+        assert sum(l.price for l in sample_lines) == Decimal("499.98")
+        assert sum(l.tax_value for l in sample_lines) == Decimal("79.83")
+    elif rounding_mode == "sum_by_gross":
+        for l in sample_lines:
+            assert l.price == Decimal("100.00")
+            # net price may vary
+            assert l.tax_rate == Decimal("19.00")
+        assert sum(l.price for l in sample_lines) == Decimal("500.00")
+        assert sum(l.tax_value for l in sample_lines) == Decimal("79.83")
+
+
+@pytest.mark.django_db
+def test_simple_case_by_line(sample_lines):
+    _validate_sample_lines(sample_lines, "line")
+
+
+@pytest.mark.django_db
+def test_simple_case_by_net(sample_lines):
+    _validate_sample_lines(sample_lines, "sum_by_net")
+
+
+@pytest.mark.django_db
+def test_simple_case_by_gross(sample_lines):
+    _validate_sample_lines(sample_lines, "sum_by_gross")
+
+
+@pytest.mark.django_db
+def test_simple_case_switch_rounding(sample_lines):
+    _validate_sample_lines(sample_lines, "sum_by_net")
+    _validate_sample_lines(sample_lines, "sum_by_gross")
+    _validate_sample_lines(sample_lines, "line")
+    _validate_sample_lines(sample_lines, "sum_by_net")
+
+
+@pytest.mark.django_db
+def test_revert_net_rounding_to_single_line(sample_lines):
+    l = OrderPosition(
+        price=Decimal("100.01"),
+        price_includes_rounding_correction=Decimal("0.01"),
+        tax_value=Decimal("15.98"),
+        tax_value_includes_rounding_correction=Decimal("0.01"),
+        tax_rate=Decimal("19.00"),
+        tax_code="S",
+    )
+    apply_rounding("sum_by_net", "EUR", [l])
+    assert l.price == Decimal("100.00")
+    assert l.tax_value == Decimal("15.97")
+    assert l.tax_rate == Decimal("19.00")
+
+
+@pytest.mark.django_db
+def test_revert_gross_rounding_to_single_line(sample_lines):
+    l = OrderPosition(
+        price=Decimal("100.00"),
+        price_includes_rounding_correction=Decimal("0.00"),
+        tax_value=Decimal("15.96"),
+        tax_value_includes_rounding_correction=Decimal("-0.01"),
+        tax_rate=Decimal("19.00"),
+        tax_code="S",
+    )
+    apply_rounding("sum_by_gross", "EUR", [l])
+    assert l.price == Decimal("100.00")
+    assert l.tax_value == Decimal("15.97")
+    assert l.tax_rate == Decimal("19.00")