Guess it is well known they had trucks, vans, tractors for baggage wagons, a pistol packing Wilma, and did business in air freight, so those are not in question.
https://penneyandkc.files.wordpress.com/2017/01/pistolpackingmamma1946.jpg
Oh, for entertainment value while waiting for time to leave to orthopedic appointment, what can be found in Google?
Hmm, wonder what the minimum radius of a Sikorsky S-40 flying boat would be. https://www.panam.org/images/igallery/resized/4701-4800/Pan_Am_Sikorsky_S40_being_loaded_with_Air_Express_cargo_at_Dinner_Key-4781-1400-900-100-c-rd-255-255-255.jpg
“Despite the fact that air express was started earlier its real development dates from August, 1934, when the entire network was linked with the domestic air express system of the Railway Express Agency, which gave more than 23,000 express stations in the cities and towns of the United States the facilities for shipping to any point in Pan American’s System. The contract between the two organizations, which runs for twenty years, is ironclad. The benefits to the two parties are mutual. On the one hand, the continents of the world are made available to the Express Agency which was formerly confined to domestic transportation. On the other hand, it has opened for Pan American the possibility for greatly enlarged intercontinental trade.”
https://www.panam.org/pan-am-stories/429-background-notes-on-pan-am-cargo
If you can get out to the Smithsonian they may have some documents which settle the matter.
Summary
Mainly forms, manuals, federal regulations, accounting instructions, and personnel materials. Included are labor agreements, an employee handbook, and information on the employees’ sick benefit association. Other materials include drawings of property owned; and photographs, engineering drawings, technical publications relating to railroad passenger car brake equipment; also a public relations publication, “The Pony Express Story.”
Cite as
Railway Express Agency Records,1898-1969, Archives Center, National Museum of American History
Repository Loc.
Smithsonian Institution, National Museum of American History : Archives Center. P.O. Box 37012, Suite 1100, MRC 601, Constitution Ave., between 12th and 14th Sts., N.W., Washington, D.C. 20013-7012. Call 202-633-3270 for appointment. Fax: 202-786-2453
http://americanhistory.si.edu/collections/search/object/siris_arc_140327