I'll throw in my limited understanding.
As I understand it, an FFL holder can ship Priority Mail(USPS) to another FFL, while I as an individual can ship to an FFL, but can not do it thru the USPS & must use Fed Ex, or UPS overnight. My FFL charges me $10.00 + actual Priority Mail cost to box & ship. That usually comes out to $20-$25 total. Fed Ex usually runs me close to $50.