Last year I got the idea to have some Christmas rubber stamps custom made for me, so I sent away for some with my own designs. Perhaps there are already some commercially available in stores but this would surprise me. Ask if stores can order them and do it in advance. With an ink pad (red or green) you can stamp greetings on any stampable surface.

The post office has no objection to "Merry Christmas" being imprinted on envelopes as long as the address isn't obstructed. The mail order company I used was "Creative Mode". You'll need to send them a black and white (no greys) image of exactly what you want made into the stamp. Do it in advance to have it ready by December.