[QUOTE=mahasamatman;9619328#4 is definitely not required. I have never signed the back of a credit card. I always write "SEE ID" on it with a big Sharpie. The only place that won't accept it that way is in person at the Post Office.[/QUOTE]

They will not accept it because your card is now VOID. 99% of places are just not as well trained in credit card acceptance. If your card is not signed it is not valid, SEE ID is not your signature, hence, the card is invalid.