Every purchase produces an invoice with a sequential number, a description of what was bought, the period, the amount and the state. The state is paid, open or overdue.
Open means the order was placed and the payment was not confirmed. Overdue is one that passed its date without payment, and the matching service becomes expired too.
Credit is a balance on the account, in cents, starting at zero. It grows with the referral commission, which is 10% of what the referred customer pays, and with entries posted by support.
At payment time you can use the whole balance, use part of it, or use none. If the balance covers the whole order, the invoice closes as paid immediately, without going through payment.