I don' think they are missing. It's just there are lots of things to be added to our streets and this is probably one of them. This is just the first test of the new system.
I'm fairly certain that even with the limited supply of test furnishings available, you should be able to transform your house into a giant mailbox. On the inside, at least.
In the new housing you won't get a mailbox. Instead, there will be a low wooden box in the corner with a scrap of old blanket, and the yoga frog will live there full time.