According to the bomb experts, PVC is too weak to create the needed compression to generate the desired explosion. Metal is needed according to the experts.
The "bomber" didn't know how to make bombs. I doubt he even considered the mailing aspect of plastic. He was just making a Hollywood prop. FYI: There are plenty of metal objects that are mailed thru the USPS.
Pipe bombs are metal for compression as you say. Also the ends are screwed on. Glued PVC caps won’t hold. The problem with mailing a metal bomb is it is far too heavy to be dropped in a mailbox. Packages must be 13 ounces or less to be dropped in a box. Over 13 have to be taken to a counter.