poppycock
USPS will ship( LONGGUNS only), from an individual to FFL as long as you fill out the form ( PS 1508) that tells them that it is going to an FFL.
And it is relatively inexpensive to boot.
On the other hand USPS only allows FFL to FFL for handguns, and it also pretty cheap.
At any rate, no matter how you send it, it HAS to go to an FFL if it is going state to state