Online prices change often, and a listed discount doesn't always mean it's the lowest price you'll see. Shopping extensions handle two separate jobs: applying available coupon codes at checkout automatically, and showing price history so you can judge whether now is actually a good time to buy.
Checking Amazon price history before you buy
Keepa overlays a price history chart directly on Amazon product pages, showing how the price has moved over weeks or months. You can also set drop alerts for a specific price, so you're notified instead of manually rechecking the page every day.
Applying coupon codes automatically at checkout
Coupon extensions scan a database of codes and test them automatically when you reach a checkout page, applying the best working discount without you searching for codes yourself. Cashback-style tools, including Capital One Shopping, can also alert you to lower prices at other retailers before you complete a purchase.
What to check before installing a shopping extension
Coupon and cashback extensions typically need to read checkout pages to apply codes, so review what data they collect. Prefer well-known tools with a clear privacy policy, and be wary of extensions that inject unexpected ads or redirect links, since some shopping tools abuse affiliate links.