Ah, but paper letters are delivered by the USPS, and thus not secure against NSA spying ether.
Metadata on a physical letter is easy to collect without evidence of tampering, and the USPS takes photos of letters to make delivery easy. They supposedly delete the images. I assume they share the images with the NSA.
The contents of a letter in an envelope are harder to spy on, because it's a manual process and you leave evidence of tampering unless you're really good and careful. But really good and careful takes even longer.
If you're a specific target, then they'll get what they want. But if you're an average unsuspected person, a letter is just too much trouble to spy on, and I think they probably don't do mass surveillance on the inside of enveloped letters.
* 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.
Much simpler.