I am a previous FFL holder and it is real easy to ship a firearm, you just need to ensure you follow federal and any state laws (I gave up on shipping anything into Kalifornia).
Step 1- create an account with fedex/ups.
Step 2- use the online system to create your shipment. I think you can even print the label at the shipping center niw days but not 100% on that. Under the cintents put "Rem 870" make and model, insure for what you want, REQUIRE AN ADULT SIGNATURE.
Step 3- drop your ready to go package off AT a hub, not a ups store/kinkos (do those even exist any more?). Take a copy of the receiving FFL with ypu IF asked. Most of the counter people have no clue what the fed, state or their business' rules are and simply turn you away or make things more complicated, have talked guns with some after dropping package off. Individuals can NOT ship handguns USPS, long guns are ok. individuals may ship a handgun or long gun to themselves care of an outfitter for hunting purposes, no FFL required.