I’ve bought 1 cent stamps from the automated postal center machines, and yes it only takes credit/debit. Sorry but spend too much time at the post office to buy that everybody in a 25 person line is ready, generally at best you’re looking at 2 out of every 10 that are actually ready to ship, and they’re in and out in under a minute.
Come to the Lynnwood, WA (ZIP 98046) postal office sometime. You’ll see that 20+ minute line, and an APC that will only sell 3 stamps at a time, minimum (unless you want to purchase priority mail stamps, or certified delivery, in which case you’re looking at $3.20+ minimum).
I use the downtown Edmonds branch - 4 miles away - which never has the line, but still has the APC limitation of 3 stamps minimum.