It is absolutely understandable that USPS does not allow their free Priority supplies to be used for other classes of mail. HOWEVER, they do not even allow us to reuse those items for other classes of mail. First of all, with free new supplies, who would want to recycle used products? When I used a USED Priority box (with the markings taped over) for a different class of mail, it was sent back to me, causing my insurance to lapse.
In another instance, I was forced to repackage a Priority envelope because I had pieces of tape on the side. I did this because I have received torn-up Priority envelopes before. But no, taping on the sides is PROHIBITED. I did not alter the package because that is prohibited. There are more stories about how postal rules will ruin your day.