I think I remember a neat trick about this.
You get the Priority Boxes free from USPS
but when you reassemble them, you do it
INSIDE OUT which is just plain brown cardboard.
just cut the glued corner and tape it back together.
that way you can put whatever postage you
want on it, and not have any problems later.
you can argue the ethics of this another day.
I wonder if there was a legal problem doing this
with the PO. not as a business, but as an individual.
I don't see any warning on the box regard misapplication
or USPS on the inside surface, like you might
on return postage prepaid envelopes.