Our wedding was fairly laid back and mostly stress free. It's a tone that I tried to set ahead of time through the invitations. Each of the four pieces was a soft touch postcard, sent out together in a custom envelope. The RSVP postcard was mailed out with a stamp already on it for the convenience of those invited.
I wanted to make it engaging both visually and physically, and doing so on a budget brought me to the back prints on each card. The "puzzle", as everyone called it, went over extremely well. A few people even commented on their disappointment in having to break up the set to RSVP.