Fix total amount calculation for event registrations with donations

aleksis/apps/paweljong/models.py

@@ -358,11 +358,12 @@ class EventRegistration(ExtensibleModel):
             )
 
     def _get_total_amount(self):
-        total, tax = 0, 0
+        total, total_tax = 0, 0
         for item in self.get_purchased_items():
-            tax += item.price * item.tax_rate / 100
+            tax = item.price * item.tax_rate / 100
             total += item.price + tax
-        return total, tax
+            total_tax += tax
+        return total, total_tax
 
     def __str__(self) -> str:
         return f"{self.event}, {self.person.first_name} {self.person.last_name}"