ditto gunsamerica and steve being in error here. the fastest way to look this stuff up is the "for sellers" links on gunbroker. i've sold about 8-10 over the last year and in spite of my putting in ads in all caps that the buyers FFL must be OK with accepting guns from individuals i still get people who don't/won't/can't read and end up with their dealers who won't do this. last time i asked "you are aware that there is no law against your receiving it from me? it's no different than if i walked into your shop and sold it to you?" "OH YEAH! we know it isn't the law. it's just our POLICY!"
so far the USPS has been 100% reliable and no problems at all. helps when people know their jobs and i always go to the same PO.
i also always go in with a copy of the recipients ffl, a copy of the on-line verification and declare what's in the box. having a handful of USGOV approved documents helps. i was in line once behind a guy mailing some long gun and he had no supporting documentation at all. they took it but didn't like it. granted, it MAY not be REQUIRED to have the license in hand but it sure helps grease the skids. it's such a simple thing to do that it's not worth being an adamant PITA just to prove a point.
roger