There is nothing in the BATF regs that state that a non licensee must have a copy of an FFL license in their possession before shipping out a firearm to that FFL. (It sure does help though!)

There is in the regs a rule that a common or contract carrier must be 'notified' that the parcel contains a firearm upon shipment of that parcel. In writing on the shipping bill is the obvious place but it does not state 'in writing'.
USPO is asking you if it contains one in their simple 'Does the package contain anything liquid, fragile, hazardous,etc' that they throw at you evertime you mail something. Pub52 in their regs.

Hard copy FFLs, FAXed copys and Emailed copys are all acceptable as proof as of 01/01/08.

A pre 1899 firearm may not necessarily be as easily shipped as a rare an precious beanybaby though we wish they were.

Some states (NJ for one) require ALL firearms to be transfered through an 01 FFL. They do not acknowledge the 'antique status' as put forth by the Fed regulations.

NY does not acknowledge the antique status of most any cartridge handguns (nor the C&R status of any) and ANY handgun shipped into NYS to a licensed dealer that would normally be registered in NYS, must come from another FFL.

Some local gov'ts have regs that do not allow the possession of certain firearms and in many cases do not exempt any by age.

An FFL not willing to supply a copy of his/her FFL copy to a non-licensee is not someone who is 'not up to speed' On the contrary it it someone who understands the regs as they are printed and that the copy is not needed by the nonlicensee.
BATFE agents on a compliance check will advise against such actions as it is well to keep a strict account of your license and where it goes. It is difficult enough to keep it from being used w/o your knowledge for all the copys that must be handed out by the regs w/o scattering more about.

With that said, I fully understand the position the nonlicensee is placed in when trying to legally ship a firearm. No one wants to be the odd man out and not have his bases covered. Everyone wants a copy of that license and only one person has it and doesn't have to give it to anyone other than another FFL.