I used Picnik.com to get a Santa Express image as well as a postmark for the envelope. I also included a small candy cane on each letter. It was easy to do and a lot cheaper than buying one for each kid. I only wished the paper was better and that I had through ahead of time to find something better.
If you want a free letter sent to your kids, send me an e-mail at staying_younger@yahoo.com, I'm willing to send out five for five of my readers.