Its partly recycling but since cards are soooooo cheap (how do they do that?), you can buy 20 cards with envelopes for about $2, so this is less about being frugal and more about doing more than " Dear Bob and Sue, Hope you have a Merry Christmas and a wonderful New Year, Best wished Jane and John Doe" each year.
I stick this generic message on the back or inside of each card to let the recipient know what I'm doing by sending them back their very own used card; Feel free to use it yourself!
Have fun and enjoy sending cards this year!!
Green-ness: 3/5 Still have to replace the envelope (in most cases!) and not every one plays the game so you still have to buy some new cards each year.
Frugal-ness: 3/5 Its not that much cheaper as I still have to buy about 20 cards each year and pay for stamps
Time cost: A lot longer than usual - each card has to have its paper cut to size, maybe an extra minute to cut and glue a new piece of paper in and find an envelope the right size for each card. However - your card list is pretty much sitting there in front of you! You'll know exactly who sent you a card last year!
Skill level: You can do this!!!
Fun -ness: When you actually get last years card back its such a fun feeling!!! There is a difference between getting a standard card and one that has crisscrossed the country several times over several years! And its wonderful to compare each year and reminisce as its added to the card!