The interesting thing about regulations is that not as many people know them who think they do. To wit...
I just happened to be mailing a technical book today at the USPS using media mail. I was watching a postal employee tape closed a flat rate priority box for a customer that obviously would not close (meaning
fit) in the standard flat rate box
without the extra tape, since what was being put into the box protruded beyond the end of the box. So the flaps didn't lay over flat.
Now I was told previously by
another postal employee that in order to be able to use a flat rate box, it had to fit within the flat rate box with no need to be taped in order to close the box. (you
could put tape on to 'secure' the package better, but it should not be needed to enclose what is being mailed)
Moral of the story? Know the regulations and be able to show them to the postal employee should they give you grief otherwise. Or go to a different PO.

And being willing to 'pay the price' based on someone down the line deciding differently.