* Prepare 100 (or whatever) onetime pads using truly random events.

* Prepare a metadata sheet indicating the serial number of each of those one time pads.

* Hand both over in a face to face meeting (you keep a copy of metadata and one time pads).

* Indicate the serial number of the onetime pad used in your letter and encode the rest of your letter using that onetime pad and post the letter.

* Destroy your one time pad and your corresponding party also destroys his/her copy in an irretrievable fashion.

This all assuming nobody else gets to see the onetime pad except you both.