Not all body bags are preprinted. Here is a priority package which fell into a mud puddle somewhere in transit. Then once in the bag, it sat quietly somewhere growing some good old fashioned mold and mildew.
The first two show two sides of the body bag:


Here while there is no printed message, the USPS did postmark the clear plastic bag in many places which all only took a faint cancellation.




This image above is the same as the prior one with a different exposure to see if the cancellation would be more pronounced. Alas, no.
This is the worst of the impact damage on this one corner, the browns are coloration from soil and the moisture (water) does not show.

This was an
ebay seller shipment of many purchases made in one day and sent in one box. The total sales were between $300-400.00. The seller made an immediate refund and was rather quickly reimbursed via insurance coverage by the USPS without hassle. sadly for me, a damaged in the mail postal history collector, this gem was just too big and too faint to be collected. The box heated the house and the bag was reused.