I have come up with a solution that seems to be working well and is appreciated.
I send the card and a clean, unused white envelope in a clear envelope (from Amber - see here:
http://www.clearenvelopesambershaw.com/ and also sold by Amber on ebay). I package it such that the front of the card is visible on the back of the clear envelope. The front of the white envelope is visible on the front of the clear envelope. Then also on the front I put a label with the address, a label with our return address and the stamp (all stickers). I do not put any extra postage on it and so far all of them have gone through without being postage due. I put the flap of the clear envelope on the front right so that it doesn't interfere with the presentation of the card and can easily be opened as its not under the return address label.
Inside I enclose a small piece of paper that says:
This is a "green" card. Remove the white inner liner and reuse the card with the enclosed envelope. Enjoy!
Inside the card, my message is written/printed on a piece of paper that is attached with temporary adhesive and easily removable.
Some people really like that they get to reuse the card. Others can't part with it