I just don't know why anyone would want one. The post office has joined forces with Looney Tunes to sell their merchandise. Post office employees seem to frequently go over the edge. Every so often one of them goes on a shooting rampage. If they are going to sell merchandise they should sell something they know, like guns.
Post offices have a separate slot for metered mail.
I have dropped stamped mail in that slot. I don't know if they ever delivered it. They probably have a big basket sitting in the back parking lot full of undelivered mail that was dropped in the wrong slot. Most post offices I have been to never seem to get the stamp dispensing machines working. Something about stamps that jam up machines.