Hm, the closest thing to the altering a box issue I've heard is you can't add on to a flat rate box and get the flat rate. Other than that, I've only been told that they have to be used for mailing only.
Deutronomy 29:29 ESV The secret things belong to the LORD our God, but the things that are revealed
belong to us and to our children forever, that we may do all the words of this law. Feedback: M98.org