Gun Broker has a tutorial on legal shipping practices, and a downloadable PDF of the USPS regs and other carriers, on the right hand side of the Home Page. Read and understand them. I also carry a spare hard copy to leave behind with both UPS & USPS staff who "think" they know the ATF Reg's ...and don't. Very Helpful.
"These are the times that try men's soul's"...Thomas Paine
"Those who fail to learn from History are doomed to repeat it" ....Santayana
"There's a sucker born every minute" P.T. Barnum, on why he always had customers to see the sideshow features.