Information from UPS about how to ship firearms, which are considered a special-care shipment and require some extra steps before packing them up and shipping.
www.ups.com
"Shipments containing Firearm Products are accepted for transportation only from shippers who are federally licensed and have an approved UPS agreement for the transportation of Firearm Products."
"UPS accepts packages containing Firearm Products for shipment only as a contractual service and only from Shippers who are licensed importers, licensed manufacturers, licensed dealers, or licensed collectors (as defined in Title 18, Chapter 44 of the United States Code) to authorized recipients, as outlined in the approved UPS agreement for the transportation of Firearm Products."
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Shippers with a Federal Firearm License
Only customers holding a Federal Firearms License (FFL) and federal, state, or local government agencies may ship firearms with FedEx. Customers holding an FFL must enter into an approved FedEx Firearms Shipping Compliance Agreement before shipping any firearms with FedEx. Also, you must be an approved firearm shipper with a signed contract on file with FedEx. For more information, contact your FedEx account executive.
Nonlicensee shippers
Nonlicensees are prohibited from shipping firearms with FedEx."
And USPS:
https://pe.usps.com/text/pub52/pub52c4_009.htm