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Yes, the test flight package is designed for test purposes and the users are selected by the developer. But this is a special mechanism and only for test flight.
As per question:
Paid for app. (Its not a free to download app)
Built and released a Test flight version.
Testflight nominated testers dont appear to be able to download and test the app without having to purchase the app.
Windows UWP app.
using namespace Windows::Services::Store;