During the war, we occasionally received letters from my cousin through the Red Cross. They were on a flimsy little fold up sheet of paper that served as letter and envelope. that you could almost see through. I'm just sorry none of those letters survived with my mom and dad.

If I recall, that was called "V-mail" and designed to limit the weight penalty of millions of letters flown back and forth between families and the troops.  I remember seeing such from my own family members but I don't know if any were saved.

ETA:  I just googled up V-mail and here's what the Smithsonian has: click here

I didn't know that the letters were microfilmed for shipping; I just thought the weight savings was in the flimsy paper used.  Score one for the government!

Stu