There is a set of various customized free stamps like Santa Stamps, Christmas Tree Stamps that you can paste on the envelope and put Santa letter inside the envelop. There are 2 types of stamps
(1) With North Pole Postmark
(2) Without Postmark
You can paste any of stamps on envelop, but Idea behind pasting postmarked stamps is to give an impression that Letter came directly from Santa who lives in North-Pole, but it is up to you.
Note: These stamps can not be used to send real postal letters.