Is there a way to add Inventory Quantities to items?

Is there a way to add Inventory Quantities to items? (for example - if I only have 5 shirts to sell -- I don't want the 6th person to be able to buy it.. )

Unfortunately, it's not possible to do that through the payment field settings, you will need to use the inventory widget instead, here is the link: https://widgets.jotform.com/widget/inventory 

You may perform calculations based on this and get the total, then you may pass the total to the payment field, here are some guides that will help you doing so: 

https://www.jotform.com/help/259-How-to-Perform-Form-Calculation-Using-a-Widget 

https://www.jotform.com/help/275-How-to-Pass-a-Calculation-to-a-Payment-Field 

I hope this helps.