# 4. Verify the result in BILLZ

> Where in the interface to see what the API did.

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.

Next — [recipes for specific tasks](/en/guides/sync-catalog/).