I think i understand how this is handled.
If you build and deploy the app in the device directly from a mac with Xcode then the payment api will take the sandbox tester you have on the client.
If you get the app from TestFlight then the payment api will take the main account of the client.
Hope will help someone
PaoloGi
Users receive reputation points by contributing quality content to the forums.
Posts include post and replies published on the forums.
Post authors can mark replies as Accepted to indicate successful solutions.
Apple Developer Forums admins can mark replies as Apple Recommended to indicate an approved solution