USPS is that last one that allows regular people to ship long guns. I heard this recently, but hoped it was a misinformed rumor. Maybe a C&R with UPS, but I didn't dig.
Reading their sites makes it look even worse. See the UPS definitions.
https://www.ups.com/us/en/support/shipping-support/shipping-special-care-regulated-items/prohibited-items/firearms.page
“Firearm Part” is any part or component of a Firearm that does not by itself meet the definition of a “Firearm.” By way of example only, Firearm Parts include, but are not limited to, barrels, stocks, grips, firing pins, sights, and magazines/clips.
“Firearm Products” means Firearms and Firearm Parts. The term Firearm Product does not include scopes.
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.
https://www.fedex.com/en-us/shipping/how-to-ship-firearms.html