Do the "Do Not Bend" Envelopes need to be sent through the post office or can they be sent directly through the post box with standard postage stamps ?
Just ensure you dont bend them before you put them in the post box....if they are A4 size you might not be able to put them in some post boxes... depending on the size of the box
When I worked as a post girl , on the Christmas post and got those "do not bend" envelopes in my bag -- I used to think" oh yes they do " , and they did !!!!