fix

alkatorapi/views.py

@@ -125,6 +125,7 @@ def register_racer(request):
 
     product.quantity -= 1
     product.save()
+
     invoice = Invoice(
         invoice_id=invoice_id,
         user=user,
@@ -147,7 +148,7 @@ def register_racer(request):
     )
     invoice.save()
     racer.save()
-    invoice.items.set([racer])
+    invoice.items.set([racer.id])
 
     payment_data = {
         'merchant': COMGATE_MERCHANT,