So in W9 I decided to set up a few shops, and one of the problems I had was keeping cupboards filled with enough money.
I would have 1 cupboard that takes all the money (and they stack when transferred to it, thankfully). Cupboards to receive the fancy sold stuff. And then there were cupboards that would take money and give it to the seller. The problem I found was this money drained out FAST.
So I'd like to suggest a metal safe/vault that has a MASSIVE storage space, but can only have coins placed inside (and hands that give/take).