Follow these steps when using a Credit on File at checkout.
Once you have all of the products in the cart scroll to the bottom of the page. Under Apply Credits select Credit on File. Now tap Add Payment.
On this screen tap on the Amount field. Enter in a dollar amount up to the amount that is shown in your Current Balance. Now tap Add Payment.
After selecting "add payment" the Credits on File will be applied to your order and your Current Balance will be reduced and will reflect the amount you applied to the order.