The toy company Melissa and Doug make a Stamp & Sort play set!  I picked this up at a local toy store as a gift for (another) friend expecting a child; it’s never too early to brain wash future philatelists!  However, if you Google “toy mailbox” you’ll find lots of cool ones.  My hope is that it will spark some imaginative play and contribute to the elaborate “playing house” narrative that lives in all little girls’ heads (or, at least my childhood was full of elaborate city life… inside a card-table play house).  Of course, if I was slightly better friends with the mom-to-be (or, her kids) I could’ve justified buy this incredible card-table play post office (pictured below).

There are quite a few folks selling toy mailboxes on Etsy, but making your own looks simple enough.