Sandboxx - Military App is an App that can assist with communicating with and sending pictures from your phone to your recruit and later Sailor when s/he is "off the grid" and only able to receive physical mail. Using the App, you can send pics in a note that includes a postage paid addressed envelope for your recruit or Sailor to write back. The cost is $2.33 to $3 per letter depending on how you choose to use the App. (See https://www.sandboxx.us/ to learn more and/or to download the App.) Some have indicated that letters sent to their recruit using the Sandboxx - Military App reached the recruit one to two days after they were sent.
I too downloaded the app, but haven't used yet. Still writing snail mail letters! But it does look like it'd be convenient for use when you want to write something quick and don't have any stationery around (like if you're traveling). I still may check it out.