This page will be useful when I sort out the ability to log in.
If you are a customer (i.e. have paid) and are making recurring payments, this page could link to a page letting you update your payment method and billing information, and see your invoice history. This is that page: Stripe Customer Portal (you can try it now). If you are a customer and have just made one-off payments then that page wouldn't be useful. This page might also be able to give some details of who you have paid for (names, recurring/one-off payments, dates).
If you are a subscriber (i.e. are taking the tests) then this page could show your subscription start date and the number of tests you've taken in the subscription period (up to 12 per year).
If you are a customer and a subscriber (i.e. you've paid for yourself) then both sets of information would be available.