I would ask to see the postal regulation that prohibits the use of a previously used shipping mailer. I've never been challenged reusing bubble mailers ... and I've re-used plenty of them.

In fact, the window clerks save the end of the roll postal meter labels for me when changing rolls in the machine. There are probably up to 30-40 or more labels on the end of the roll prior to changing rolls ... you know; the ones with the pink strip at the top? I use them to cover used mailing labels that won't peel off and I affix postage stamps to the blank meter label.