Here is the latest Letter-to-Santa reply envelope (2014)

Not very celebrated in the philatelic world, these envelopes have brought a reply from Santa Claus every year since 1982.
After 2005 the indicia have been the current secular Christmas stamp (like regular postal stationery) and pre-cancelled at the North Pole (like commemorative envelopes)
For the last two years, however, Mrs. Claus has graced the corner.
Oops! I hear that Tony Shaman's booklet (previously mentioned) is no longer in print!