Although it hasn't been a secret that's been kept by the postal system, mailing any item that's less than 108" in combined length and girth doesn't require a box or envelope. It's a small fee (depending on the size) but the possibilities are endless.
Over at The Giver's Log, they've been busy cataloging the items they've mailed in the past, in fact they've even dedicated a Flickr pool to such things and although most items are just for fun, they could easily be transformed into an invitation of symbol of holiday well wishes. Besides, who wants a card hanging around when you could have any number of things that might have actual purpose instead of sitting there and looking pretty.
Try printing your invitation or greetings on sticker paper and attach them to something simple like a box of candy. If you're feeling daring, use an old pop bottle and fill it with assorted treasures, an actual invitation, glitter, snacks or photos. Think outside the box and check out a few ideas over at The Giver's Log.
Image: The Giver's Log