It is well known phrase to be used on collector's mail: "do not bend" , to much the mail containing postcards or philatelic stamps or books or banknotes or photos. I suggest people to use it on mail thoughtfully.
Why? Because it attracts unhealthy attention of potential theifs who may expect that cash is enclosed, which may finally cause the loss of mail or damage of it. Or potentially evil people may intentionally bend exactly this mail to make harm, while unmarked mail goes through with no attention.
The best alternative to this marking is strong packing which makes bending physically difficult.
Think of it!
Finally, if You still decide to write/affix on Your mail the note "do not bend", then do it both sides,so then it is more chance that the postman reads it!