If you have Store Credit on your account and you would like to apply towards your purchase, you can do so by clicking on the 'Apply Store Credit' link in the Cart screen.
In the pop up box, you can select the amount of credit you would like applied to the order.
The Store Credit cannot be used on shipping charges or tax. Also, Store Credit cannot be used toward subscription payments.
If you have any further questions, please email support@zno.com.