This is something the U.S. Postal Service offers every year. It allows holiday letters and cards to be stamped with "Santa" as the sender.
The deadline is Monday, Dec. 10. To take advantage, you need to place the card or letter from Santa in its envelope with first class mail stamps.
You should also have the return address as "Santa, North Pole."
Place the card in a larger envelope with the correct postage and address it to:
North Pole Postmark Postmaster
4141 Postmark Drive
Anchorage, Alaska 99530
You can get more information about USPS' Letters from Santa program by visiting www.usps.com.