Handguns may be shipped by an individual, but must be shipped overnight by UPS of Fedex. Long guns may be shipped by an individual through USPS, UPS or Fedex.
Dealers can mail a handgun through USPS.
Some dealers require that an FFL send any gun, that is because they don't know the rules. There is no state or federal law requiring that a gun be shipped by a dealer, it is the practice of the carrier for a handgun only. Not every gun dealer is a genious.
UPS's policy came about because their employees were stealing the handguns, not a federal requirement. So because they employee thieves, the public has to pay for it.
If a USPS clerk tells you you can't ship a long gun, ask to speak to the postmaster, because that employee does not know their policy, seen it many times. If they give an anti-gun opinion, file a complaint with the postmaster. It is against their policy to spew anti-gun propaganda.
My mother is a post master.
I have a dealer friend of mine mail handguns, much cheaper than overnight by UPS. I mail every long gun that I sell over the internet. If the gun is shipped within the state, no FFL is required.