Credits. If you have a credit balance on your account, the credit will be applied to your next invoice(s) until it is used up, though it might not look like it is being applied until the last step or so in the PayPal Checkout process, but you will see it on the Invoice, and it is applied. The exception to this is when you are using a PayPal Subscription (Automantic payment profile), credits do not get applied, rather you would need to first cancel the PayPal Subscription using the cancel button, then pay the Invoice using the PayPal Checkout button, and the credit would then be applied. Once all the credit amount has been used up, simple set up another PayPal Subscription from then on.