Once inside your subscription portal, scroll down to the section called 'BCause Bucks'. This is where you will see your current store credit balance. To have your balance be used on your next subscription order, click this toggle to on.
When toggled on, the system will use all of your store credit when processing your next order. Any balance remaining will be charged to your card on file.
If you would like to save up your store credit, make sure to leave this toggle in the off position.