Unloaded rifles and shotguns may be mailed if the mailer fully complies with the Gun Control Act of 1968 (Public Law 90-618) and 18 U.S.C. 921. The mailer may be required to establish, by opening the parcel or by written certification, that the gun is unloaded and not excluded from mailing because of the restrictions in 431.2b and c.
... but I wouldn't recommend it.
They lost my package (no, it wasn't a gun).Why not - what happened the last time you shipped via USPS?