Click on "order status/account" - go to "view my gift cards" - record the gift card code to input during checkout. If you have a coupon, there is an enter coupon code box at the top left of your cart. Enter coupon code and click the "validate" button.

If the other answers dont work or you still have problems click on the blue chat now button and they will help you (9am-9pm ET 7 days a week)

During the purchase process there will be a place to add a coupon code with an apply button around the payment screen