Your available store credits should be reflected at the bottom of the "Shopping Cart" page.
Please make sure you are logged in to your account, press on the "Check out" button to proceed, and the "Return to Cart" button if it is not reflected on the page.
You should then be able to see the available store credits on the "Shopping Cart" page and the available amount will be automatically deducted from your total.
Please contact us at hello@themono.co if you require assistance in utilising your store credits.