The Post Master we had here for several years, would not let anyone other than an FFL holder, ship long guns and no amount of arguing would change his mind. He wouldn't even allow long guns to be sent back to authorized repair centers, despite having the paperwork in hand explaining how to send them back.
I don't believe USPS will ship handguns, unless both parties have an FFL, if even then?
Most around here would go to another town to ship long guns via USPS, because the PM over there actually understood USPS rules. Now our former PM, is the PM in that town.
No problem here since he left. I was gonna ship one of the rifles I sold on here last fall and the babes at the PO counter, gave me two of their triangular Priority Mail shipping tubes for free, to box it up in.
FWIW, most dealers I know, use UPS. And when I got firearms from CMP, they used FedEx.