It is against the law & the USPS will fine you if you are caught. I'm sure many get away with it, but eventually you'll get caught. I'd really hate to see anyone here get fined for such a small thing [esp. when no one hardly has jobs anymore]. :(  I'm not trying to be a tattle, nasty or anything else, just trying to help you all out. :)

To clarify that - you cannot use them inside out on any mail class, but what they are marked at. You CAN re-use Priority boxes on Priority mail. So no using them on Media Mail and First Class packages (which is pretty much no using them on PBS mail)

The USPS doesn't fine you, but they do mark the package up to the rate it would have cost had it been shipped Priority.  The receiver can refuse delivery due to the postage due and the book will come back to you. No credit and you will need to pay the postage due to get the book back. Not really worth the gamble in my opinion.

People had a "heydaY" with those until the USPS started the "No inside out use" for other levels of delivery rates rules.  They were such a good product to use and re-use.  But we can't afford it on media mail.  I wish I could use their Tyvek envelopes! 

Ruth