On the checkout page there is a box on the right side of the screen where a coupon/discount code or a gift card number can be entered. It is located in the order summary section between the items ordered and the order subtotal.
If you are using a mobile device, once on the checkout page, click on "Show order summary" at the top of the page to reveal the details of the order as well as a box where you can enter a discount code or gift card.
