How to setup an order form through calculations.

create a subtotal based ONLY off of the "quantity" box (am I able to create a calculation value in the subtotal box only?).  Because currently the user would have to check the box (which seems redundant) AND add a number in the quantity box for a subtotal to calculate.

You could set this up through a payment field, or Purchase Order field. They have the option to setup quantities: https://www.jotform.com/help/165-Setting-Up-Your-First-Order-Form 

If you want to do it through separate fields, you can simply multiply the amount by the quantity:

Also, just to let you know, you can automatically check the option if the quantity field is filled: https://www.jotform.com/help/268-How-to-Insert-Text-or-Calculation-into-a-Field-Using-Conditional-Logic 

Result: https://form.jotform.com/90468786695984 

Let us know if you need more help.