diff --git a/server/vbv_lernwelt/core/models.py b/server/vbv_lernwelt/core/models.py
index f1ee630d..71d4a3db 100644
--- a/server/vbv_lernwelt/core/models.py
+++ b/server/vbv_lernwelt/core/models.py
@@ -118,7 +118,7 @@ class User(AbstractUser):
     # is only set by abacus invoice export code
     abacus_debitor_number = models.BigIntegerField(unique=True, null=True, blank=True)
 
-    def set_increment_abacus_debitor_number(self):
+    def set_increment_abacus_debitor_number(self, disable_save=False):
         if self.abacus_debitor_number:
             return self
 
@@ -130,7 +130,8 @@ class User(AbstractUser):
             current_max if current_max is not None else 60_000_000
         ) + 1
         self.abacus_debitor_number = new_debitor_number
-        self.save()
+        if not disable_save:
+            self.save()
         return self
 
     def create_avatar_url(self, size=400):
diff --git a/server/vbv_lernwelt/shop/migrations/0015_cembra_fields.py b/server/vbv_lernwelt/shop/migrations/0015_cembra_fields.py
index 67061756..b714a6cd 100644
--- a/server/vbv_lernwelt/shop/migrations/0015_cembra_fields.py
+++ b/server/vbv_lernwelt/shop/migrations/0015_cembra_fields.py
@@ -39,4 +39,9 @@ class Migration(migrations.Migration):
             name="phone_number",
             field=models.CharField(blank=True, default="", max_length=255),
         ),
+        migrations.AddField(
+            model_name="checkoutinformation",
+            name="refno2",
+            field=models.CharField(blank=True, default="", max_length=255),
+        ),
     ]
diff --git a/server/vbv_lernwelt/shop/models.py b/server/vbv_lernwelt/shop/models.py
index 1272d881..8373bc4d 100644
--- a/server/vbv_lernwelt/shop/models.py
+++ b/server/vbv_lernwelt/shop/models.py
@@ -62,6 +62,7 @@ class CheckoutInformation(models.Model):
 
     invoice_transmitted_at = models.DateTimeField(blank=True, null=True)
     transaction_id = models.CharField(max_length=255)
+    refno2 = models.CharField(max_length=255)
 
     # end user (required)
     first_name = models.CharField(max_length=255)
diff --git a/server/vbv_lernwelt/shop/services.py b/server/vbv_lernwelt/shop/services.py
index b83091c9..f5998d38 100644
--- a/server/vbv_lernwelt/shop/services.py
+++ b/server/vbv_lernwelt/shop/services.py
@@ -7,6 +7,7 @@ import structlog
 from django.conf import settings
 
 from vbv_lernwelt.core.admin import User
+from vbv_lernwelt.shop.const import VV_PRODUCT_NUMBER
 from vbv_lernwelt.shop.datatrans.datatrans_api_client import DatatransApiClient
 from vbv_lernwelt.shop.models import CheckoutState
 
@@ -77,6 +78,7 @@ def init_datatrans_transaction(
     redirect_url_error: str,
     redirect_url_cancel: str,
     webhook_url: str,
+    refno2: str,
     datatrans_customer_data: dict = None,
     datatrans_int_data: dict = None,
     context_data: dict = None,
@@ -90,6 +92,7 @@ def init_datatrans_transaction(
         "currency": "CHF",
         "language": user.language,
         "refno": str(uuid.uuid4()),
+        "refno2": refno2,
         "webhook": {"url": webhook_url},
         "redirect": {
             "successUrl": redirect_url_success,
diff --git a/server/vbv_lernwelt/shop/views.py b/server/vbv_lernwelt/shop/views.py
index 67b8db4d..3d0cbad5 100644
--- a/server/vbv_lernwelt/shop/views.py
+++ b/server/vbv_lernwelt/shop/views.py
@@ -13,6 +13,7 @@ from vbv_lernwelt.shop.const import (
     VV_DE_PRODUCT_SKU,
     VV_FR_PRODUCT_SKU,
     VV_IT_PRODUCT_SKU,
+    VV_PRODUCT_NUMBER,
 )
 from vbv_lernwelt.shop.models import CheckoutInformation, CheckoutState, Product
 from vbv_lernwelt.shop.services import (
@@ -115,13 +116,16 @@ def checkout_vv(request):
     ip_address = request.META.get("REMOTE_ADDR")
     email = request.user.email
 
+    request.user.set_increment_abacus_debitor_number(
+        disable_save="fakeapi" in settings.DATATRANS_API_ENDPOINT
+    )
+
+    refno2 = f"{request.user.abacus_debitor_number}_{VV_PRODUCT_NUMBER}"
+
     try:
         datatrans_customer_data = None
         datatrans_int_data = None
         if with_cembra_byjuno_invoice:
-            if "fakeapi" not in settings.DATATRANS_API_ENDPOINT:
-                request.user.set_increment_abacus_debitor_number()
-
             # see https://api-reference.datatrans.ch/#tag/v1transactions as reference
             datatrans_customer_data = {
                 "firstName": request.user.first_name,
@@ -155,6 +159,7 @@ def checkout_vv(request):
             ),
             redirect_url_cancel=checkout_cancel_url(base_redirect_url),
             webhook_url=webhook_url(base_redirect_url),
+            refno2=refno2,
             datatrans_customer_data=datatrans_customer_data,
             datatrans_int_data=datatrans_int_data,
             context_data=context_data,