Suka's reply is only relevant to the Glock.
The AR, you can mail (USPS) to the company, and they can mail it back to you. No FFL need be involved. You are allowed to mail longarms to yourself, to FFLs, and to places of repair.
NEGATIVE!!!!!!!
an AR lower is a firearm must ship to an FFL.
An 01 FFL is required to work on firearms for profit. Mailing a firearm across state lines is interstate commerance , whereas, they are collecting money from you to refinish the Lower.
Since the receiving Gunsmith is not "yourself" one cannot mail it to "yourself". which also the company is not a 01FFL. It is a technically a out of state transfer, which requires a FFL on both ends.
ATF FAQ:
Q: Is a license needed to engage in the business of engraving, customizing, refinishing or repairing firearms?
Yes. A person conducting such activities as a
business is considered to be a gunsmith within the definition of a dealer.
[27 CFR 478.11]
GCA:
.............may ship a firearm to a
licensed importer,
licensed manufacturer, or
licensed dealer for any lawful purpose, .................... the
licensed manufacturer,
licensed importer, or
licensed dealer may return in interstate or foreign commerce to that person the firearm or a replacement firearm.........