A stripped lower is a firearm and but does not meet the legal definition of a either a rifle or shotgun. Although it's always been defined this way the ATF has made this clear with the revised 4473. AFAIK as an unlicensed person you can only use the USPS to ship a legal length rifle or shotgun.
The USPS also reserves the right to open a parcel to establish that the gun is unloaded. Unless you give notice that the package contains a gun how could you comply with this regulation?
http://pe.usps.com/text/pub52/pub52c4_009.htm
d. 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 432.1b and c.