If a credit is issued to your Avery account you will only be able to view this amount from within your account during the checkout process. On the Review & Payments section, you will be able to select Use Store Credit below the Payment Information box.
Note: store credit will not appear if there is a mix of products in your shopping cart, such as having both Blank Labels and WePrint Sheets. You will need to create separate orders to use the store credit on your account.