How can I retrieve customer payment info once submitted?

The credit card numbers cannot be retrieved. When the total amount is $0.00, the payment details are not being sent to the payment gateway.

If your goal is to collect valid payment details, you may consider using a Stripe integration in the authorization mode. With the Stripe integration in authorization mode, you can collect valid credit card details (e.g. authorize $0.5 USD) and charge custom amount payments through your Stripe dashboard later. Note that you will not see the full credit card numbers.