When I got busted by the PO for reusing a Priority Box for NON Priority use, they said they were doing this for that reason. These boxes are free, people were taking them, turning them inside out and using them for cheaper mailing fees.
If the PO notices the PRIORITY words under a brown paper wrapping, or the words while xraying/scanning them...and you have NOT payed the Priority fee, they will return the package to you, and then you have to pay AGAIN to remail it.
The PO told me that the only way to recycle these boxes is in the recycling bin :roll: