Adding items to GUI slots

I'm trying to add items to my GUI slots, like a store basically. 

I made a wallet item, with its own GUI,  so when you right click it brings it up and has slots to put your money/coins in. (almost like a backpack)

I want to add a button that says "Store" and when its clicked it opens another GUI that has slots filled with items that you can buy for currency. 

Only problem I am having is I can't figure out how to add items to the slots that stay when after you close the store. 

I also want it to randomize the items every time you open it so it's not just the same items over and over. 

I've checked out the "How to make GUI with slots" video but it didn't help me.