Envelope scans are required to enter the prize draw for the $10 gift certificate. It's just to ensure you didn't just print the picture and sign in yourself, or use an old one, just to win the prize. If you don't want to enter the draw, by all means don't bother posting one. They were trying to help you out.
http://www.fanmail.biz/mboard/viewtopic.php?f=2&t=398
Personally, I found your post rude too, but since I'm just a regular member and not a moderator, I didn't have the position to say anything. I'm sure Larisie wasn't the only person to think so. Please remember that these guys are doing this in their free time

as a hobby, they don't get paid to do this and they aren't required to do anything for you at all. It's just something they do to help out the fanmail community, and personally I'm very glad they do {thumb2}
Either way, congratulations on the autograph. David has been wonderful to his fans, despite his agents request that fanmail be sent to the ITG address.