We're actually going to be doing a mail carrier/mail unit study here next year, so this is going be useful for us, besides just being fun.
The way I see it, there are two routes you can go for this kit. For an older child, I'd put together an actual mail kit, complete with a book of stamps, some fun stationary, some stickers, a cool pen, and a list of relevant addresses, (grandma's, a favorite cousin's, etc.)
I may in fact do that anyway for L, who is just now getting interested in getting real mail.
For little ones, like V, a toy felt set seems like a better idea. Here's the simple set I put together for mine (constructed of felt, and machine sewn,) but if you're feeling a little more ambitious, here's a great tutorial for a more elaborate mail set.
I was going to try to construct a felt mailbox, and then thought better of it. I think we still have a couple of little tin mailboxes lying around here from Valentine's day.