[palmer]

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.

[kabar]

You can mail packages using the automated self-service kiosk in the post office. The maximum size that will fit in a self-service kiosk drop off is 20 3/8″ x 15″ x 12″.