They definitely X-ray parcels, for security. Shortly after 9/11, I remember a local post office failing to detect explosives and it getting reported on; this was notable for two reasons:
- one of the hijackers had received a knife through the mail, and it had been processed through that office, so there was extra scrutiny at the time
- a few years prior, that office processed and delivered a package the Unabomber sent that killed someone
The majority of mail screening these days is to find illegal drugs. However, given that so many drugs get through, it seems they only x-ray a small subset.
Yes, the USPS started scanning packages and letters since the great Anthrax scares. This is just a productization of an already widespread, in-use technology.
- one of the hijackers had received a knife through the mail, and it had been processed through that office, so there was extra scrutiny at the time
- a few years prior, that office processed and delivered a package the Unabomber sent that killed someone