If you have a coupon code you'd like to apply to your next order you can do so by entering the code in your Billing Details.
Step 1: Click "Billing Details" tab under the "My Account" menu found on the left-hand menu if on a desktop or on the drop-down menu if on a mobile device.
Step 2: Scroll down to find the "Redeem a coupon" section.
Step 3: Enter the coupon code in the box.
Step 4: Click "Apply"