Recently I bought some stamps from the post office vending machine. Unlike all other times in the past, this time the stamps were not the colorful ones of the type you get when you buy a book from the counter. They were black and white, printed on the vending machine, and they have the zip code of the place of purchase on them. As well as some other numeric identifier
which would probably match up with your purchase transaction.
This was a first for me personally.
So if you have OPSEC concerns (e.g. battered wife in hiding or whatever), consider buying your books of stamps from the counter or the supermarket checkout cashier.
I'll bet the Kansas BTK killer wishes he had paid a little better attention to OPSEC, instead of sending his final letter in a digital format which when you clicked on the file properties, had info which went right back to his work location....