How do I take this subtotal calculation and add it to the payment wizard at the bottom of the form to get a grand total?

Also, how do I take this subtotal calculation and add it to the payment wizard at the bottom of the form to get a grand total?

Unfortunately, you cannot pass a custom calculation to the payment wizard that has products setup on it. 

If you want to pass a calculation value to the payment wizard, you need to change the "Payment Type" from "Sell Products" to "User Defined Amount". You can learn more about this on this guide: https://www.jotform.com/help/275-How-to-Pass-a-Calculation-to-a-Payment-Field 

With that, you'll need to recreate the existing products on your payment wizard to the default form fields in the form. 

I hope that helps.