After you get bonus store credit, you can use it for any future orders. All you need to do is to input the amount you wish to use in the "bonus store credit" column. Please refer to the picture below.
Note: If you just want to use bonus store credit alone for your order, in the precondition of sufficient bonus store credit balance you have, you can click any payment method and then click the “Continue” button. It will direct you to the order successful page.