Fix tax calculation issue

src/pretix/base/models/orders.py

@@ -1874,7 +1874,7 @@ class OrderFee(models.Model):
             self.tax_rule = self.order.event.settings.tax_rate_default
 
         if self.tax_rule:
-            tax = self.tax_rule.tax(self.value, base_price_is='gross', invoice_address=ia)
+            tax = self.tax_rule.tax(self.value, base_price_is='gross', invoice_address=ia, force_fixed_gross_price=True)
             self.tax_rate = tax.rate
             self.tax_value = tax.tax
         else:
@@ -2026,7 +2026,7 @@ class OrderPosition(AbstractPosition):
         except InvoiceAddress.DoesNotExist:
             ia = None
         if self.tax_rule:
-            tax = self.tax_rule.tax(self.price, invoice_address=ia, base_price_is='gross')
+            tax = self.tax_rule.tax(self.price, invoice_address=ia, base_price_is='gross', force_fixed_gross_price=True)
             self.tax_rate = tax.rate
             self.tax_value = tax.tax
         else:

src/pretix/base/models/tax.py

@@ -188,7 +188,7 @@ class TaxRule(LoggedModel):
         return Decimal(self.rate)
 
     def tax(self, base_price, base_price_is='auto', currency=None, override_tax_rate=None, invoice_address=None,
-            subtract_from_gross=Decimal('0.00'), gross_price_is_tax_rate: Decimal = None):
+            subtract_from_gross=Decimal('0.00'), gross_price_is_tax_rate: Decimal = None, force_fixed_gross_price=False):
         from .event import Event
         try:
             currency = currency or self.event.currency
@@ -200,7 +200,7 @@ class TaxRule(LoggedModel):
             rate = override_tax_rate
         elif invoice_address:
             adjust_rate = self.tax_rate_for(invoice_address)
-            if adjust_rate == gross_price_is_tax_rate and base_price_is == 'gross':
+            if (adjust_rate == gross_price_is_tax_rate or force_fixed_gross_price) and base_price_is == 'gross':
                 rate = adjust_rate
             elif adjust_rate != rate:
                 normal_price = self.tax(base_price, base_price_is, currency, subtract_from_gross=subtract_from_gross)

src/tests/base/test_orders.py

@@ -2512,7 +2512,7 @@ class OrderChangeManagerTests(TestCase):
         assert nop.price == Decimal('23.00')
         assert nop.tax_rule == self.tr19
         assert nop.tax_rate == Decimal('100.00')
-        assert nop.tax_value == Decimal('19.33')
+        assert nop.tax_value == Decimal('11.50')
 
     @classscope(attr='o')
     def test_change_taxrate_from_reverse_charge(self):

src/tests/presale/test_checkout.py

@@ -427,7 +427,7 @@ class CheckoutTestCase(BaseCheckoutTestCase, TestCase):
         cr1.refresh_from_db()
         assert cr1.price == Decimal('19.33')
 
-    def test_country_taxing(self):
+    def _test_country_taxing(self):
         self._enable_country_specific_taxing()
 
         with scopes_disabled():
@@ -450,6 +450,25 @@ class CheckoutTestCase(BaseCheckoutTestCase, TestCase):
 
         cr1.refresh_from_db()
         assert cr1.price == Decimal('23.20')
+        assert cr1.override_tax_rate == Decimal('20.00')
+        assert cr1.tax_value == Decimal('3.87')
+        return cr1
+
+    def test_country_taxing(self):
+        cr1 = self._test_country_taxing()
+
+        self.client.post('/%s/%s/checkout/payment/' % (self.orga.slug, self.event.slug), {
+            'payment': 'banktransfer'
+        }, follow=True)
+
+        self.client.post('/%s/%s/checkout/confirm/' % (self.orga.slug, self.event.slug), follow=True)
+        with scopes_disabled():
+            assert not CartPosition.objects.filter(pk=cr1.pk).exists()
+            o = Order.objects.last()
+            pos = o.positions.get()
+            assert pos.price == Decimal('23.20')
+            assert pos.tax_rate == Decimal('20.00')
+            assert pos.tax_value == Decimal('3.87')
 
     def test_country_taxing_free_price_and_voucher(self):
         self._enable_country_specific_taxing()
@@ -490,7 +509,7 @@ class CheckoutTestCase(BaseCheckoutTestCase, TestCase):
             assert o.positions.get().price == Decimal('23.20')
 
     def test_country_taxing_switch(self):
-        self.test_country_taxing()
+        self._test_country_taxing()
 
         with mock.patch('vat_moss.id.validate') as mock_validate:
             mock_validate.return_value = ('AT', 'AT123456', 'Foo')