The US Postal Service will postmark letters from Santa. You are responsible for writing the letter. Put the letter in a stamped envelope, and then put that inside another stamped envelope and mail it to: North Pole Christmas Cancellation, Postmaster, 5400 Mail Trail, Fairbanks, AK 99709-9998. The deadline is usually 15 December. My youngest loves it.
Update: The links at the bottom of the page are saying that the letter program has ended. This one hasn’t. This letter is from you, the postal service is just returning it, cancelled and postmarked. No potential pervs involved.