Fix negative prices in bundles when tax rate is 0 (#4513)

src/pretix/base/models/tax.py

@@ -306,8 +306,11 @@ class TaxRule(LoggedModel):
 
         if rate == Decimal('0.00'):
             return TaxedPrice(
-                net=base_price - subtract_from_gross, gross=base_price - subtract_from_gross, tax=Decimal('0.00'),
-                rate=rate, name=self.name
+                net=max(Decimal('0.00'), base_price - subtract_from_gross),
+                gross=max(Decimal('0.00'), base_price - subtract_from_gross),
+                tax=Decimal('0.00'),
+                rate=rate,
+                name=self.name,
             )
 
         if base_price_is == 'auto':

src/tests/presale/test_cart.py

@@ -3654,6 +3654,31 @@ class CartBundleTest(CartTestMixin, TestCase):
         assert cp.price == Decimal('0.00')
         assert b.price == Decimal('1.50')
 
+    @classscope(attr='orga')
+    def test_voucher_apply_multiple_reduce_beyond_designated_price_no_tax_rules(self):
+        self.ticket.tax_rule = None
+        self.ticket.save()
+        self.trans.tax_rule = None
+        self.trans.save()
+        cp = CartPosition.objects.create(
+            event=self.event, cart_id=self.session_key, item=self.ticket,
+            price=21.5, expires=now() + timedelta(minutes=10)
+        )
+        b = CartPosition.objects.create(
+            event=self.event, cart_id=self.session_key, item=self.trans, addon_to=cp,
+            price=1.5, expires=now() + timedelta(minutes=10), is_bundled=True
+        )
+        v = Voucher.objects.create(
+            event=self.event, price_mode='set', value=Decimal('0.00'), max_usages=100
+        )
+
+        self.cm.apply_voucher(v.code)
+        self.cm.commit()
+        cp.refresh_from_db()
+        b.refresh_from_db()
+        assert cp.price == Decimal('0.00')
+        assert b.price == Decimal('1.50')
+
     @classscope(attr='orga')
     def test_voucher_apply_affect_bundled(self):
         cp = CartPosition.objects.create(