For the majority of users, the verification process via Google Play might work like charm.
But for those (including me) who have quit with google, this might be a bit harder.
Fist of all, I've purchased Poweramp in the first instance from the google play store, years ago. But after a time, I've deleted my google acc, which wasn't a problem as Poweramp has been already registered.
Some weeks ago, I had to restart my phone multiple times and after that, Poweramp requested a purchase verification to the google play services (which of course weren't able to verify the purchase).