There [STRIKE]may be [/STRIKE] IS a difference in laws regarding shipping of rifles vs pistols.
FIFY
USPO laws/regs state that anyone can ship a long gun to factory.
USPO laws/regs state that ONLY DEALERS can ship handguns to factory (or anywhere else). [NOTE: C&R FFLs are NOT dealers and can't use this option.]
"Common Carriers" (i.e. UPS and FedEx DEPOTS, not stores) can ship any gun to the factory. Since UPS and FedEx have employee theft issues, they implemented "rules" that mean we get screwed (financially) on shipment of handguns by being "required" to ship by air only. This is NOT a law, but a rule of each carrier. Mfrs & distributors frequently use 2nd Day Air, so that "should" be available to commoners too, although be prepared for an argument on that one.
From personal experience:
When you go to a depot with a sealed box, pre-paid shipping label already attached and tell them it contains a gun (most of us agree that this is required by law, although some disagree), they will treat you like a professional business person and accept the shipment without BS.
If you are paying to ship the gun, create an online (free) account with the shipper and create the pre-paid shipping label. Be aware that most mfrs do NOT use their real name on the shipments to further minimize theft (e.g. S&W uses "SWEC" IIRC), so ask the mfr what to put down.
ALWAYS print out that part of the carrier's tariffs (available online) that cover shipping of firearms and bring it with you. That stops a lot of "made up rules" by clerks that are clueless.
By doing what I stated above, I have NEVER been challenged or had any problems with shipping handguns (multiple times) or long gun (once) at UPS or FedEx depots.
ETA: If you go in with an unsealed box, no pre-written shipping label, and ask them what to do . . . you will be met with all sorts of BS such as . . . you must have an FFL, you must disassemble the gun, you must ship the most expensive overnight air, etc. Message is act professional and get treated as a professional who knows what they are doing . . . act clueless and you'll be treated accordingly.