Authorize.net: How do I calculate total quantity purchased in my form?

I am using a form add on that is not allowing me to calculate "Quantity" totals at check out.

As I can understand you need to get the total quantity purchased in your payment field: 

Unfortunately, you cannot get each quantity field from your payment section, if you need to get a total quantity of elements purchased then you will need to manually set up your products in your form and then use calculations to pass the total amount to the payment field, this way you will also be able to calculate the quantities purchased and get a total. 

Here's a guide that will help you to pass a calculation to the payment field: 

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

And this form will help you to perform calculation in your form: 

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

If you have any question, let us know.