Most high value firearms dealers are self insured and ship UPS. I suspect it's simply a a matter of UPS pick up versas USPS going to a Post Office.
That said, my one experience shipping UPS resulted in the case enclosing a Pigeon Grade Superposed Superlight being splintered on one interior corner near the trigger guard dispite the reciever being nicely wrapped in bubble pack. Trigger guard on Superposed was OK. Lesson, don't use UPS and don't ship within a case, use standard heavy duty box and wrap very well in bubble pack. I use USPS Registered for high value and have had perfect success.-Dick