This information won't be useful for about six weeks, as Elder Kennington won't be in Mexico until mid-September, but I wanted to get it posted right away to make sure I didn't forget.
Sending pouch mail is a little different than regular mail. Senders can mail a letter that will be delivered to Elder Kennington in Mexico for the cost of a first class U.S. postage stamp, but the guidelines are very strict about what can be sent:
--One-page correspondence written on sturdy paper (notebook or other lightweight paper will not process through USPS machines) written on one side of the paper only
--Anything added to the folded over letter
"Items not complying with the ... instructions will be returned to the sender, or discarded if the complete return address is not included." -- Elder Kennington's missionary packet instructions.
In order to send the letter for the courier to take to Mexico, here are the instructions as given in his packet:
Directions for mailing the letter
"Lay the letter blank side down. Fold the bottom of the letter about one-third of the way up the page and crease. Fold the top of the letter to the bottom of the first fold and crease. Secure the long side with two pieces of tape about one inch in from each end, but do not seal the ends. Write your name and complete return address in the top left corner. Affix first class postage in the top right corner. In the middle, write the missionary address..." I've posted his pouch mail address to the right.