If there is available credit balance for the reseller account, it will appear as a default payment option on the payment screen.
You can choose not to use the balance for the current payment by unchecking its box. If the credit amount is not enough to cover the required payment, you will have the option to submit just the difference between the available credit and the required payment.
Payments for hosting services with the account balance are without VAT. If applicable, VAT will be added to your balance clear or refill payments.