Per the USPS Dim Weight guidelines from the MSP, they state "If you use an existing box, it must be the correct size and CANNOT HAVE ANY MARKINGS ON IT EXCEPT THE TO/FROM ADDRESS FOR THIS SHOP". So if you are not covering the size you are not following the shop guidelines.
As for the IRS, underreporting your income is fraud, end of story. If you choose to look at it as a gray area, that is your choice. Either way, it causes the IRS double work which in itself is wrong. Most posters whine and complain when an MSP causes them extra work, this is no different.
**edited to add the word 'to"
Edited 1 time(s). Last edit at 03/30/2016 12:01AM by grille.