we bought the postcards and the stamps at the shop there. There is someone there who puts the Hogsmeade postmark on the postcard---we actually had the TM stamp that first, then we wrote out the postcard (at the TM's suggestion, that way we knew how much space we had left to write on the postcards). Then we brought them back to the TM and and the TM dropped them in the mail slot. We sent one to ourselves and you can see the "real" post office post mark (that says Orlando and the date) lightly over the Hogsmeade Owl Post mark.