4. Verify the result in BILLZ

At the start of an integration, every API call is checked against the BILLZ interface.

Action through the API	Where to look in BILLZ
Creating and paying a sale	Sales → All sales of the selected shop, the receipt by date and amount
Adding a product to a draft	the Drafts section of the shop
Changing a custom product field	product card, characteristics tab
Creating a supplier	supplier directory
Write-off, transfer	the corresponding warehouse section — the process appears in the list with a status
Changing a customer’s cashback	customer card, balance history

Checklist before going to production

The token is refreshed automatically.
All asynchronous calls either wait for the response with the Billz-Response-Channel: HTTP header or check the result with a separate read request.
429 errors are handled with a retry and a pause.
A 408 response does not lead to a blind retry of the write: the object state is checked first.
Catalog synchronization: incremental (last_updated_date) once an hour, full — once a day.
The secret key is stored in a secure vault.