There is one post office in my area that will not let you pay at the counter with a credit card. They are nuts. You can do the whole interaction and be ready to pay, and they will stop and walk you to the SSK. I have shopped no less than 100 Post Offices in multiple states and only this one location is weird like that.
Sometimes the tellers wear red vests too I've noticed. Are they in compliance when they do that?