To/From you to a FFL holding gunsmith is legal.
However, as a private person, it depends on state and other regs.
For example, handguns go overnight, at around $90, to ship out from me.
My LGS can ship it from him to the same gunsmith for about $16. However, it NOW has to come BACK to him.
So, I can have it shipped more directly, and incur a big expense, or I can have my LGS do it.
Note: my favorite LGS for this lets me just ship from him, and just charges me shipping/materials fee [box/tape/bubblewrap]- $16.
When gun returns, he just calls and I come get it.
most other LGSs will charge something like $50 to handle it for me. I am lucky that I have options. Even in California.