It's a bag. Four slots. 500 gold. Each one you buy increases the price by 500 gold. Why? It can hold other bags. Like bags with things in them.
Infinite bags.
Bag in a bag in a bag in a bag in a bag full of other bags.
So I can have a bag in my bag when I open my bags.
Bagception.
Call it The Bag of Holding Bags.
People would stop whining about not having enough bag space and start whining about having too many bags.
And I would be okay with this.
Think of all the coin purses we threw away.