There is a workaround:
Install the app being logged in under real Apple id
Create a sandbox email
Sign out from the real Apple ID
Sign in using Sandbox email instead of real apple id
Open the app ( you don't need to use TestFLight!). Just open as an app from the screen.
Pay from the app
Open Settings - App Store - Sandbox - Manage - see your subscriptions
(NOTE! to update the app to the newest version you'd need to log in back with real apple id, upload new version, repeat steps 3-7)
JuliaKiseleva
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I signed in under ***@ ***.com Apple ID
In the meantime, I'm connected to App Store> Sandbox ID is yyy@ yyy.com (which was registered via "Users and Access" > "Sandbox Testers" tab)
I go to the TestFlight app and pay
When I confirm payment, there is a modal ask to enter a password from ***@ ***.com ( But it must be yyy@ yyy.com )
I go back to App Store> Sandbox ID and see that yyy@ yyy.com is now changed to ***@ ***.com
I click manage, and it says Cannot Connect.