Most places I deal with will ship long guns UPS but use FedEx for handguns.
This is taken from the UPS website:
(l) All shipments, including shipments containing firearms, will be accepted for transportation only according to the terms and conditions contained in the UPS Tariff. A UPS label, requesting an adult signature upon delivery, must be affixed to each package containing a firearm. Access the Terms and Conditions of Service at ups.com or contact UPS for more information concerning the transportation of firearms, including handguns.
Transportation of Firearms, including Handguns
(m) UPS reserves the right to assess the shipper an additional charge, set forth in the UPS Rates in effect at the time of shipping, when: the shipper requests UPS to obtain a signature or an adult signature upon delivery and an approved UPS label is affixed to the package indicating such request; the shipper requests UPS to obtain a signature or an adult signature upon delivery and an approved UPS label is not affixed to the package indicating such request; or, the shipper tenders a package that, based upon its contents, requires an approved UPS label requesting an adult signature upon delivery, and no such label has been affixed to the package.
(n) Shippers are prohibited from shipping and UPS will not accept for transportation packages containing any Hazardous Materials requiring shipping papers (defined as those materials regulated under Title 49 of the Code of Federal Regulations [49 C.F.R.]), firearms or ammunition, or articles that UPS is not authorized to accept or that UPS states in the UPS Tariff or in these Terms and Conditions of Service that it will not accept, when such packages are presented for shipment at a UPS Authorized Shipping Outlet or UPS Commercial Counter. Hazardous Materials (except ground ORM-D) cannot be picked up via one-time pickup or UPS On-Call PickupSM service, or retrieved via any UPS Return Service.
(o) UPS cannot deliver to a P.O. box. The shipper must make every effort to obtain a street address. If the shipper should use a P.O. box address, the recipient’s phone number must be included. A package addressed to a P.O. box may experience delays, will not be covered by any UPS Service Guarantee, and will be subject to an Address Correction charge. Army Post Office (APO) and Fleet Post Office (FPO) addresses are not accepted.