I used a Sharpie to write addresses on the side and got busy installing them around the play area.
Used a (free after rebate) level to keep them straight.
Used a drill bit to make holes in the 2x4's that I mounted them in.
Then to make holes in the mailbox.
I made 1 hole attached it to the 2x4, then drilled the second hole to ensure that I didn't move it in between.
Since I couldn't fit a drill inside the mailbox and I didn't want a sharp point from a screw on the inside, I chose to use 1 3/4 inch machine screws and nuts to bolt them in place. (2x4's aren't actually 2 inches thick)
Used blank note cards and stickers to make mail. The kids have already been making their own mail, so that should be fun to see evolve as time goes on.
I even found some small boxes to wrap and address. I covered it all in clear packaging tape to help them live a bit longer. I also made sure the kids knew the boxes where empty.
Now the kids can deliver and receive mail by matching the letters and numbers.