I would like to buy the Android app but I’m using CopperheadOS, which does not include Play Services / Play Store. Is there any way I can buy this app?
No, actually not.
See here
https://community.cryptomator.org/t/will-the-direct-apk-download-option-be-updated/550/3
Yes it is possible to get paid app on custom rom, just install yalp store from f-droid, log in to your google account and then in browser log in on your google account and buy this app. This way app will be available to download via Yalp Store app
And it’s also possible to pay using Google Play gift card, as i did 