How can I add an option to pay with cash?

 On this form I want to allow the user to be able to submit the form and pay cash when they arrive, how can I make this happen?

You could use "Purchase Order" widget. 

You can check the guides here to understand the order form basic and types.

https://www.jotform.com/help/47-Order-Form-Basics

https://www.jotform.com/help/49-Order-Form-Types

Guide on how to create your "Purchase Order" payment type. https://www.jotform.com/help/460-How-to-Create-Payment-Form-with-Purchase-Order