Fixed #23 -- Tax handling for payment fees

src/pretix/base/migrations/0011_auto_20160311_2052.py

@@ -0,0 +1,26 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.9.2 on 2016-03-11 20:52
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0010_orderposition_secret'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='order',
+            name='payment_fee_tax_rate',
+            field=models.DecimalField(decimal_places=2, default=0, max_digits=10, verbose_name='Payment method fee tax rate'),
+            preserve_default=False,
+        ),
+        migrations.AddField(
+            model_name='order',
+            name='payment_fee_tax_value',
+            field=models.DecimalField(decimal_places=2, default=0, max_digits=10, verbose_name='Payment method fee tax'),
+        ),
+    ]

src/pretix/base/models/orders.py

@@ -123,6 +123,14 @@ class Order(LoggedModel):
         decimal_places=2, max_digits=10,
         default=0, verbose_name=_("Payment method fee")
     )
+    payment_fee_tax_rate = models.DecimalField(
+        decimal_places=2, max_digits=10,
+        verbose_name=_("Payment method fee tax rate")
+    )
+    payment_fee_tax_value = models.DecimalField(
+        decimal_places=2, max_digits=10,
+        default=0, verbose_name=_("Payment method fee tax")
+    )
     payment_info = models.TextField(
         verbose_name=_("Payment information"),
         null=True, blank=True
@@ -157,8 +165,18 @@ class Order(LoggedModel):
             self.assign_code()
         if not self.datetime:
             self.datetime = now()
+        if self.payment_fee_tax_rate is None:
+            self._calculate_tax()
         super().save(*args, **kwargs)
 
+    def _calculate_tax(self):
+        self.payment_fee_tax_rate = self.event.settings.get('tax_rate_default')
+        if self.payment_fee_tax_rate:
+            self.payment_fee_tax_value = round_decimal(
+                self.payment_fee * (1 - 100 / (100 + self.payment_fee_tax_rate)))
+        else:
+            self.payment_fee_tax_value = Decimal('0.00')
+
     def assign_code(self):
         charset = list('ABCDEFGHKLMNPQRSTUVWXYZ23456789')
         while True:

src/pretix/base/settings.py

@@ -57,6 +57,10 @@ DEFAULTS = {
         'default': 'True',
         'type': bool
     },
+    'tax_rate_default': {
+        'default': '0.00',
+        'type': decimal.Decimal
+    },
     'show_items_outside_presale_period': {
         'default': 'True',
         'type': bool