It's probably not something you'd want to prioritize, and you mentioned releasing it quickly influenced some of your decisions. But a way to pay outside of the PlayStore would be nice for those of us who chose to or cannot use it for payment.
Google wont allow distribution via Google Play, and payment via third party I think.
Threema recently introduced their app, which requires a paid-for license, through F-Droid using their own repository and license shop parallel to their established offer on GooglePlay.
I can only echo rchaud in assuming in theory it should be possible to sell the license and do the check independent of Google like desktop software has been doing for decades.
Whether this is something worth your time is, of course, a totally different question.
Just wanted to make you aware of this in case you weren't.