Yesterday I took a large envelope, 10-1/2 x 12, to the postal substation nearest me to be mailed to PA (I'm in SWFL).  I had printed "Do Not Bed" on the front of the envelope as there was something inside I didn't want ruined.  The postal clerk told me that because of the "do not bend", the envelope would have to be sent at a parcel rate of $6.00.  I asked for the envelope back, scratched out the "do not bend" and got it mailed for $2.10.  Big difference in price I'd say.

If you don't want something to be folded or bent, it needs to be packaged with a stiffener so it can't be folded or bent. Then it will be rigid, so USPS will class it as a parcel, rather than a flat, and it will cost more. It's their system.