Keepa is a shopping-focused tool that tracks and compares Amazon products to make sure you always buy them at the best price.
After choosing which country you're buying your Amazon products from, you can choose between two options: using the app's built-in search engine, entering the key-word of a product that you want to buy, and starting comparing products and prices, or browsing straight from Amazon itself (which can be accessed from within the app).
However, when searching from within Keepa itself, you can see all the online store's products, find out price history, set alarms for when a product you're interested in goes down in price, get more information about each product, and scan products and barcodes to search for the same item on Amazon and find it at a better price. In this sense, the app has similar features to price comparison apps, except that here, you're not comparing with other stores, but just product price fluctuations on Amazon.
Keepa is a good complement for your purchases, letting you find out each product's value so you can decide if it is a good moment to buy, or if it is worth waiting a little more.
