Hello @Ingrid, I appreciate your wish to support OGS with one-off donations (even though I personally prefer to give a small amount regularly so that our devs can (somewhat) extrapolate what might come in over time).
You may want to read this thread:
IIRC you are not the only one who prefers one-time payments instead of “subscription”, and (also IIRC) @anoek and @matburt are working on a system to take such payments into account. (Please forgive my “IIRC” but my memory is like a sieve 
and you’ll be able to read up more info in the linked thread.)
Greetings, Tom