How can I identify that an item is sold out and make it unclickable?

I'm sorry, but we do not have an inventory feature in our payment fields. You'll have to identify it manually and disable the item once it is sold out.

If you wish to do it automatically, you have to do it through our widgets, as explained from the following guide:

https://www.jotform.com/help/426-How-to-Setup-a-Payment-Order-Form-with-Limited-Stocks