I try to find the bigger birthday cards at the dollar store. They need to be at least 5" wide. Then I make equal stacks of candies and set them inside the cards. Once you close the card and slip it into an envelope, those candies won't move. There is no need to cover them with plastic wrap, as many people like to do. This makes a card that is thin enough to slide through the slot the P.O. uses to check them with. I don't remember how much the postage costs, but will check when I go to the P.O.
Additional hint: If you prepare the cards beforehand and leave them in a pile under a book, they flatten even more.
So is our swap to start in March?