How to tally up totals for paypal based on previous form conditions

I'm trying to build a registration form with a paypal checkout at the end. 

Ideally the tally would be there based on what they had filled out previously (the amount of ppl they register). 

How do I do that?

I have inspected your form Willowdale Soccer League and if I understood you correctly you want to pass your form calculation to your PayPal payment field. 

Inside of this guide you can find how to do it:

How to Pass a Calculation to a Payment Field

I also was that your form does not have PayPal payment field, Inside of this guide you can find how to integrate your form with PayPal:

How to Integrate Form with Paypal

Hope this will help. Let us know if you need further assistance.