I personally would like to see all cards sent naked as postcards for this exchange, AS WAS SUGGESTED FROM THE START. We will know the numbers sent and the numbers received, and once and for all know if the instant post card is possible.
I think that if one pays full postage (letter rate not postcard rate) to cover extra handling, that it would almost be "tampering with the mail" for postal workers to throw away private mail. They may not like it, they may discourage it, but to throw it away, I don't know about that.
I vote to stamp them and send them, be done with it.