Hello.
At the moment, this is not implemented in the integration.
But in the API description
https://dev-api.checkbox.in.ua/api/docs#/%D0%A7%D0%B5%D0%BA%D0%B8/create_receipt... there is an option to specify the is_return parameter when transferring products, which implies apparently that this is a return of the product.
We can refine the setting in the receipt creation action that a return must be performed, and accordingly, when sending a request, the above parameter will be passed to each product.
If you need it - let us know.