^^ this. even cursory x-ray inspection is a huge undertaking.

I once worked for the USPS in a Middle Eastern country, and it was our job to assist local customs officials as they scanned / inspected every piece of inbound mail.

They ran each piece through an x-ray, and whenever they spotted a banned item (liquor bottles, sex toys--hell, they even inspected optical discs to make sure they weren't porn), they opened the package and confiscated it.

We cleared about 1 mail truck of mail every night, and it took 5-10 hours. No way that's sustainable on the scale of domestic mail throughput.