Like this, but was told to post in ideas. So the idea is to be able to buy some donate version via PayPal, BC, Bank,. .. whatever sees fit, in case you're using a (example) google-free setup.
There are already illegal methods out there, so I doubt it would cause damage, only benefit those wanting to support the Dev. Could help Google users too, as I've read about diff mail acc and other probs with Pro app
About how, I am not sure, examples:
Xprivacylua standalone (website, you get an app-internal link that unlocks pro app. Only one semi-automatic, depending on payment option, more initial, less work afterwards)
Titanium Backup (email, licence key you put in storage)
Afwall donate (email, link to unlock app, though updates unclear)
Nova Launcher Prime (iirc through app, then: pp, email, licence code registered in app, a bit fuzzy for c&p code, did save in keepassdx then)