Long guns can be shipped USPS by anyone that can legally own one, they'll only accept handguns from licensed dealers tho. Their shipping cost is considerably less than what UPS and Fedex charge, and it's faster, I recently shipped an H&R Ultra Slug Hunter, shipping weight was 10lbs, cost was $26.20 insured priority mail to zone 6, his FFL had it in 2 days. The only time I use UPS is if I have to, either at the request of the recipient or I'm shipping something than can't be shipped by USPS like ammo.
One thing you need to know about shipping to an FFL is many FFL's won't accept a shipment from a non-licensee, they'll only accept shipments from other licensees(FFLs), that needs to be clarified with the buyer before you ship, otherwise when they receive it, they won't transfer it to the buyer and it ends up being a very expensive transaction.
Tim
http://pe.usps.com/text/pub52/pub52c4_009.htm