Is there an automated way of adding a quantity field that also updates total cost?

In your form in integrated to our Payment Tools, you can add a quantity option by the following the steps from this guide:

http://www.jotform.com/help/225-How-To-Add-Options-In-The-Payment-Form (I. Add Pre-Defined Quantity Property).

But I see that your form is configured to collect payments from individual textbox/calculation fields so, in this case, you will need to setup the quantity using Dropdown field and use a formula in the calculation.

Thanks