You can keep all sorts of things in ziplock bags, including stamps and addresses (and in another piece, postage in coins!). One of the rules of the postal system is that they aren't supposed to open your mail, but in order to cancel this stamp they did just that. By "cancel", we mean of course that they scribbled on the stamp with a ball point pen. Also inside was a "secret message" taped in the corner. After this piece was posted on the website, it got "sent again"