Express Mail Tyvek Mailing Envelope
Image by Aranami
Customers were cheating and turning the Priority Mail and Express Mail packaging inside out so they wouldn’t have to pay the premium rates. This caused the Post Office to print "Priority Mail Postage Required" in black text on certain easy-to-flip forms of packaging, such as the Priority Mail DVD Box.
Recently, in 2010, this spread to nearly every form of free Priority or Express packaging provided by the Postal Service. This was first printed on the insides of the long rectangular boxes, then spread to the other boxes, and finally began to appear on the inside of the Tyvek envelopes. The lettering in the Tyvek envelopes and on older boxes is usually printed red; newer boxes will have alternating dark blue text and red text (as if to prevent people from carefully marking out every word with colored magic marker).
There are a few variations on the text itself. Black text printed inside the specialized Priority Mail boxes (DVD, and Book/Cassette) will only say "Priority Mail". The red or alternating red/blue text inside mailers will usually say "Priority Mail Postage Required", and may also have "It’s time to rethink your mailing with Priority Mail" or "Thank you for using Priority Mail service".
Note that the inside of most Express Mail boxes and envelopes will have anti-cheat printing that mentions Priority Mail, not Express Mail. They are probably printed on the same line and use the same cardboard stock.
So far, the triangular "tubes", the shoe box and the flat-rate envelopes do not have inner printing. Express Mail packaging is slower to be used up so many Post Offices have older stock without inner printing (for all you cheaters). These boxes are not easy to flip, but you can try.