Have you guys seen this? Or nobody really cares lol i just thought it was a bit interesting
"X-ray all incoming mail. (Large mail centers.)"
http://about.usps.com/securing-the-mail/best-practices.htm
Its under the General Safety and Security Procedures 2/3 of the way down the page
It's part of their procedure. Has been for ages. The substances you might receive won't be suspicious from an xray. It only reveals in detail dense objects, like metal or bone.