Private citizens can/do use US Mail (allowed per USPS regs) to ship long guns, but USPS will only allow the mailing of handguns by FFL's.
That said, if/when I have to mail a handgun, I go to a Post Office and obtain (free) a FLAT Rate Box of the appropriate size to fit my handgun, and take the package to my FFL for mailing. The USPS Flat Rate max's out @ $15, and a friendly FFL may trot it to their PO or let his PO Delivery person pick it up on the cheap.
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