How do I store items in the extra inventory slots?

[isolated1]

I purchased the case to store more items and guns but when I buy a gun from the vendor in the game, it lets me place it in the extra inventory case, and then I am not sure what key is used to save it. The only key I tried that does anything is the esc key, but when I hit it the 2 menu options it gives me, it exits the inventory without saving the item, and I try to buy it again from the vendor, and the same thing happens. I have no problem placing guns in the part of the inventory that you start the game with, but that is full so I need to know what key to save guns in the right side case so I can get more guns. Thanks.

[Tomberry Blue]

The grid "outside" of the inventory when you buy an item while your inventory is full is just that, outside. It's a temporary space so you can sort your stuff out.

When you buy the space upgrade the suitcase gets slightly bigger, it's pretty easy to miss it.