Cently, formerly known as Coupons at Checkout, is a Chrome extension that finds coupon codes for thousands of online retailers and shows you the best ones at checkout. Cently also has a feature called Amazon Best Price, which tells you when a product is cheaper from another Amazon seller.

Understand why codes don't work. A code may have expired, may not be applicable to specific brands, may not be transferrable or may only be able to be used once. It's also possible your purchase didn't fulfill the requirements of the code, such as spending a certain amount.