I'd like to add a pay button to my form to allow my tennis students to purchase credits and subsequently use them. How best to track?

I am not sure if I understand your query correctly. Do you mean to track the credits purchased using the form? For purchasing the credits you may be able to setup an order form using a payment field. Please refer to the guide below that can help you with creating an order form.

https://www.jotform.com/help/165-Setting-Up-Your-First-Order-Form

We request you to provide us with more information on your requirement of tracking credits so that we can check it further and see what best we can do in this regard.

Thanks!