Instead of enabling one-time donations, I see some value in sponsorships with a predetermined EOL: Instead of donating $10 one-time, get a 10-month sponsorship for $1 each and at the end of those 10 months, decide whether you want to continue supporting the developer. Github could charge you the $10 (or the interest-adjusted equivalent) up-front to decrease the transaction costs too.
This decreases the variation in the developer's income and lets them plan for the future accordingly, which is always a good thing.