Handguns are not mailable. A common or contract carrier must be used to ship a handgun.
Are you sure about that?
An FFL holder may ship handguns via USPS.
As for the USPS not asking about whether a firearm is being shipped, they generally don't. But ...
Publication 52 - Hazardous, Restricted, and Perishable Mail 432 Mailability
432.1 General
Mailers must comply with the Gun Control Act of 1968, all of the provisions of postal law in 18 U.S.C. 1715, and all other all federal and state regulations and local ordinances affecting the movement of firearms. The following also applies:
a. The Postal Service may require the mailer to open parcels containing firearms or air guns or give written certification that the weapon is unloaded and not concealable.
I've never had a USPS employee ask specifically what is in my package or if it was a firearm. Only the typical, is it hazardous yadi yadi spiel. If they asked what is in it, I like an answer someone once posted here on the subject: machined parts. And hope they wouldn't press further. Since I package rifles with a lot of care, I'd be more likely to walk out and find another PO to ship from than open the package for inspection, because getting it out and back in would be quite the hassle.
Another USPS link with more info:
DMM Revision: Mailing Firearms — Clarification
