Inventory on products

I am in need of help. 

I have one product that I am using PayPal for checkout.  I have the CC and shipping figured out but am still not sure how the inventory widget works. I need to keep track of how many DVDs are sold. 

I also need help with form calculation for using a check. If the visitor wants to purchase with check, how can I calculate the shipping just like I have it setup for CC. and I will need inventory on this too. 

Is there a better way to do this? 

You do not have an Inventory widget in your form. Do you want to use this widget?

I inspected current form settings and if I understood correctly, you also want to perform a calculation inside of the form when Check option is selected. 

Here is a demo form that I created - https://form.jotform.com/82272092145958 

Feel free to test it and clone it. 

Will this work for you?

Inside of the following guides, you can find how to perform calculation inside of the form:

How to Perform Form Calculation Using a Widget

In my form removed your product option form the payment field and I am passing the calculation from the form to the payment field. 

How to Pass a Calculation to a Payment Field