As of Dec 16, 2020, I'm using Gumroad to handle all my flatsale adopts not preclaimed on Patreon. This allows me to provide better customer service even though I have the memory of a goldfish cracker.
This method keeps me protected from eating paypal's high chargeback fees and prevents the trouble caused by everyone sending me their shipping address as though they're buying a physical good.
Buyers get their crop/fulll res files right away! Also, I usually cross-post, and now nobody will need to wait for me to reply if a design is available or not.