API: New implementation for cart creation (#2833)

src/pretix/api/serializers/order.py

@@ -1086,6 +1086,10 @@ class OrderCreateSerializer(I18nAwareModelSerializer):
 
                 seated = pos_data.get('item').seat_category_mappings.filter(subevent=pos_data.get('subevent')).exists()
                 if pos_data.get('seat'):
+                    if pos_data.get('addon_to'):
+                        errs[i]['seat'] = ['Seats are currently not supported for add-on products.']
+                        continue
+
                     if not seated:
                         errs[i]['seat'] = ['The specified product does not allow to choose a seat.']
                     try: