When you put a letter in the box, it is hardly touched by human hands. The letters get dumped in a machine that faces all the letters so they can be fed into other machines that read the addresses and sprays a barcode on it. It does no good to write anything on the envelope, as the computer is only looking at a small portion of the envelope where the address should be. If you give it to the clerk at the window, and tell her to hand cancel it, it will not go through the machines and get bent.