Many of the posters have been rubbing the nub, but aren't quite there yet. I'm not sure I can get there either. The real question is whether an AR lower is a rifle, handgun or stripped lower. The ATF has added stripped lower classification to a FFLs permanently bound book (or computer records) specifically for unassembled lowers. The end point selling FFL, upon receipt, can log the receiver in any of these 3 classifications. Obviously, a receiver logged in as a rifle can only be built as a rifle. A receiver logged in as a pistol can be built as a pistol (or possibly as a rifle). The stripped lower has no restrictions outside the NFA. Theoretically, a stripped lower logged in by the receiving FFL as a rifle, then built as a rifle can never be built (or rebuilt) as a handgun. There are specific Federal laws concerning this. I believe a handgun can be converted to a rifle as long as the NFA is complied with. A stripped lower would seem to be the best option because it can be built as either a rifle or handgun and converted to either as long as the NFA is complied with. The rub comes from how does your state feels about this. Some take the tough road and determine that since a logged stripped lower can be built as a handgun then their petty handgun rules apply. I think most don't care one way or the other as long as Federal guidelines and rules are obeyed.
As far as I know all these rules and determinations are still kinda up in the air. A contradictory ruling could come into effect in a heart beat (see Sig Arm Brace rulings). I don't know how the USPS feels about all this. I kinda feel it would depend on what classification the FFL you bought it from logged it in as. That's a tough cookie to crack. I'm sure it would require court time and a subpoena. I would also expect the USPS to take the conservative stance and determine on lowers logged in as rifles only can be shipped by individuals. So what ya gonna do? Drag your dealer down to the PO with his bound book to prove your lowers are rifles? They were prolly logged in as stripped lowers.
Edit: In cases where I've used stripped lowers substitute receiver only.