How do you ship a gun through USPS?

Depends on the gun. A rifle or shotgun that is unloaded MAY be mailed in the US Mail. See Domestic Mail Manual, section 14, restricted mailings. A handgun may only be mailed by a licensed dealer or manufacturer. If the OWNERSHIP of the gun is changing, other than by inheritance, and it is going to another state, the gun must be sent to a licensed firearms dealer in the state of the receiver of the gun.