only FFLS (this does not include C&R FFLs) can
MAIL a handgun via USPS.
only the USPS is "mail"...remember, words mean things, mail is USPS, not other shippers...it pays to be precise here
YOU can ship the handgun via UPS/Fedex or any other carrier that allows it, most have restrictions on from WHERE they accept handguns, like at main hubs only, not UPS stores and such.
be aware that under federal law you must tell the carrier that there is a firearm in the package, you cant list it as "machine parts" ir "precision hole puncher" or something else.
also note that some FFLs are jackasses and only accept guns from other FFLs, check with the buyers FFL for thier restrictions.
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Flectere si nequeo Superos, Acheronta movebo."
—Virgil
"Tact is for people not witty enough to use sarcasm"