POST OFFICE design !!!

I'm developing a universe around the profession of... Postman.
Yes, yes! The hero of my comic strip is a small postman, or at least he decides to become one when he literally falls into this post office!!
He will find thousands of undelivered letters from twelve years ago!
Some contain incredible mysteries that could change the face of the world... Or at least the lives of the recipients!
So he decides to take them with him!
And here is the place where he will stock up between each mission! It's an absolutely gigantic post office, inside a giant turtle carcass, but that's not enough! The holes originally for the animal's legs now open into galleries dug into the canyon rock and the ground!