Look for the checkout page, where your credit card details are asked for. Scroll down to the heading DEBIT/CREDIT CARD. Immediately above this heading is a link that says ENTER A COUPON OR PROMOTIONAL CODE. If this link does not appear, the option you’ve chosen is non-eligible for further savings.
To redeem towards a rental car, from the homepage, click on CARS, then ADVANCED OPTIONS. A number options will drop down. Choose your rental car company, and enter your discount code.
To redeem a coupon towards a bundled vacation, carry on to the REVIEW AND BOOK page. You will seee the link underneath the Payment options.
There is no box provided on Expedia to redeem a flight coupon. Check carefully and you will usually see that the deal you were offered has been automatically applied.