So this is more inspiration than a tutorial, but I thought I'd be fun to share it. I had the idea to create a sort of cabinet. First I wanted to do drawers, but thankfully I realized it would be WAY too complicated, so I let that go.
For this snailmail I used an old (A5) mailbox-box, from which I cut the front loose to create a door.
The 'slats' that create the sections are taped in with wide tape. They have the width of the box so I could slide them in and same height so they wouldn't stick out. I worked from large to small.
Next I secured them in the box, so they couldn't slide back and forth.
After that I just went nuts with tape, stickers and stamps.
The bottom (large) section is for the letter and the rest for goodies.