I copied this from the Seller's Firearm Shipping Guide on GunBroker's website:
(B8) May a nonlicensee ship a firearm by common or contract carrier?
A nonlicensee may ship a firearm by a common or contract carrier to a resident of his or her own State or to a licensee in any State. A common or contract carrier must be used to ship a handgun. In addition, Federal law requires that the carrier be notified that the shipment contains a firearm and prohibits common or contract carriers from requiring or causing any label to be placed on any package indicating that it contains a firearm.
[18 U.S.C. 922(a)(2)(A), 922(a) (3), 922(a)(5) and 922(e), 27 CFR 478.31 and 478.30]
(B7) May a nonlicensee ship a firearm through the U.S. Postal Service?[
A nonlicensee may not transfer a firearm to a non-licensed resident of another State. A nonlicensee may mail a shotgun or rifle to a resident of his or her own State or to a licensee in any State. The Postal Service recommends that long guns 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. Handguns are not mailable. A common or contract carrier must be used to ship a handgun.
Here is the link to the entire page:
http://gunbroker.com/Support/SupportFAQView.asp?FAQID=1118&NoCount=1
I've shipped a few long guns using the USPS and had Zero problems. I will add that I always told them I was shipping a shotgun or rifle. The only thing they asked was if it was loaded. I said no it isn't and that was that. I always took along the FFL copy when I mailed the item, and was never asked to see it or anything like that. Then I just kept the copy for my records.