This is an incredibly sweet idea. "Open when you need to know how much I love you; open when you don't feel beautiful; open when you need a laugh; open when you miss me; open when you're mad at me; open when you need a date night, etc.
What a nifty idea! if you want creative envelopes, go to Maps, map the route from your letter to the other person's mailbox. Print them up, fold them into 8 by 11 envelopes (or smaller if you are sending cards). This would be cool for pen pals
This is a great idea not just for pen pal! Put the puzzle together.write the message.take it apart.mail in sturdy envelope. Small inexpensive Christmas scene puzzles would make great holiday greetings!