Probably violates the terms of service for developers. I suggest looking for the developers social media and see if they have donations available there.
Just haven't been caught yet maybe. Google and Apple have been in the news recently for not allowing references to external income sources within apps listed in the play/app store.
After a quick search I couldn't find any other way to support the dev (unless it's on their Twitter, which I found, but couldn't see it unless I logged in, which I didn't do)