There are two ways to redeem a gift card, through the checkout page or through the account page.
Redeeming a gift card on the checkout page
If you want to redeem a gift card on the checkout page simply copy the card code from the gift card notification and paste it into the coupon code field. You can also see this gift card code by going to Issued Cards
Issued Cards shows the card code on the list page as well as the details page.
Copy the code received and past into the coupon code section of the checkout page.
Note: Shipping & Tax is calculated on the total before the gift card was applied. The gift card can also cover to the cost of shipping and tax.
If you do not want to use your gift card on checkout hit 'Disable' next to the gift card.
Redeeming on Account Page
Log into the customer account page and you'll see a new section showing current gift card balance. Don't see this? Make sure you've installed the gift card macro.
Select Redeem Gift Card and enter the gift card code. The customer will now be able to use this gift card on checkout automatically.