I see someone likes their coffee. Good example of recycling!
This person send me cards folded up in a standard sheet of paper. While this does provide some bulk to the envelope, I would still reccommend at least putting the cards in a penny sleeve first to help with potential moisture.
This one is a clever use of cardboard. Additionally, you can see that this was sent in a secutiry envelope. This type of cardboard, as you can see from the pictures, is quite thick. If you want to mail cards this way, don't forget to add extra postage! This card is certainly more than 1/4 inch thick.