From FAQ's on ATF site
http://www.atf.treas.gov/firearms/faq/faq2.htm#f13
F14) Who may ship handguns through the U.S. Postal Service? [Back]
Federal firearm licensees may send an unloaded handgun in the mail to another FFL in customary trade shipments. Handguns also may be mailed to any officer, employee, agent, or watchman who is eligible under 18 U.S.C. 1715 to receive pistols, revolvers, and other firearms capable of being concealed on the person for use in connection with his or her official duties.
However, postal service regulations must be followed. Any person proposing to mail a handgun must file with the postmaster, at the time of mailing, an affidavit signed by the addressee stating that the addressee is qualified to receive the firearm, and the affidavit must bear a certificate stating that the firearm is for the official use of the addressee. See the current Postal Manual for details.
The Postal Service recommends that all firearms be sent by registered mail and that no marking of any kind which would indicate the nature of the contents be placed on the outside of any parcel containing firearms. (See also Questions B7 and B8.)
More on the subject that states collectors CAN'T use the USPS.
http://proofhouseinc.com/PostalRegulations.htm