I'm pretty sure most postal works have no use for trading cards. If they are tampering with PWE it's probably because they think it's giftcards.
Definitely not worth losing your job and freedom for baseball cards.
If the postal worker is both corrupt and has knowledge of the hobby, then they'd probably figure that cards being sent in for grading are worth something.
Besides, when dealing with criminals, don't expect them to follow what the average person considers to be "common sense."