I shipped a rifle yesterday, USPS. I showed up with it already packed and boxed. I told him it was a rifle when I handed it to him, and he said - (incorrectly) you can't ship that here. It has to go UPS.
I was about 98% sure on this, but who knows what got tacked on to some piece of 399-1 legislation in the last year, so I asked if I could see the regs prohibiting it. After pointing out the legal difference in long guns and hand guns as it relates to the USPS, he said, ok, but he wanted me to write that it was unloaded on the box. I showed him that part (no markings on the outside indicating that it is a firearm), and he took it.
A couple points in summary:
Be polite, be patient.
UPS an Fedex will take long guns, handguns, machine guns. But box store shippers quite often won't. You will seriously fry some hardwired brains if you try to ship from Mailboxes Etc. Take it to a major office, a real post office, or a shipper that you know or have reason to believe will not freak out. (Local pawn shop is a UPS shipper. I use him when I need to send out a gun.)
If they tell you no, politely ask to see something in print telling you why not.
Bring tape with you just in case. Make sure that you pack it well - if it has room to get a running start inside the box, it might try to escape, and we all now how dangerous firearms running loose in the Post Office can be.