Any credit available in your account will be subtracted from the shopping cart total after clicking the blue Checkout button, but before you enter any credit card information.
In the shopping cart, the subtotal in the Overview does not reflect the credit:
However, click the blue Checkout button to see any Available Credit in your account reflected in green within the Order Summary:
You can always click the "Back to shopping cart" button to update or make any changes to your shopping cart before entering any credit card information. Happy shopping!