Box it carefully as if you'd expect gorillas to handle the sorting of packages on a moving escalator.
Be prepared to show a copy of the receiving FFL's license if mailing out of state. Address must be the same on the license and the 'mail to' block of the address label. Include a photocopy of your GADL or GWL for the receiving FFL's records. The postal clerk may ask to see the copy inside the package (I doubt this has ever happened as it presupposes that the clerk would have some knowledge of actual postal regulations related to shipping long guns via the postal service) so bring tape to reseal the package if you have to open it.
Person to person within the State, no documentation is required.
Be prepared to show a copy of the receiving FFL's license if mailing out of state. Address must be the same on the license and the 'mail to' block of the address label. Include a photocopy of your GADL or GWL for the receiving FFL's records. The postal clerk may ask to see the copy inside the package (I doubt this has ever happened as it presupposes that the clerk would have some knowledge of actual postal regulations related to shipping long guns via the postal service) so bring tape to reseal the package if you have to open it.
Person to person within the State, no documentation is required.