Not an expert, so I try to play it safe.
Suggest license to license only whenever shipping, especially interstate. So, FFL/C&R to FFL/C&R is fine for C&R eligible firearms, although some state laws may prohibit certain transfers.
For example, a CZ82 pistol is C&R eligible, but with its 12-shot magazine, illegal in some states (like CA). If transfering a C&R handgun
within my state, I have to go through a state licensed FFL dealer or the state police and observe the waiting period - yet I can do an interstate transfer direct.
When shipping handguns, licensed FFL
dealers can ship via USPS, but it's fuzzy (to me) regarding C&R status, so I suggest a commercial carrier like UPS who will accept as a second-day air shipment. I believe all carriers accept C&R long guns.