More and more retailers use automated computer programs that can rapidly change prices in response to competitor pricing, supply, demand and other factors. The practice is called dynamic pricing. Online e-commerce companies have been at it for years. Now, with the adoption of electronic shelf tags and displays, it can spread through the brick-and-mortar world too. I’ve seen digital price tags at my local supermarkets, Best Buy and Kohl’s. But prices can be fluid even without dynamic pricing. What you pay can depend on when and where you shop, for example, and what deals you get.

The best way to beat fast-changing prices is to be flexible on the timing of purchases so you can pounce at just the right moment. For that, you need price trackers and alerts, which help you follow changing prices whether you shop online or in stores. Here are some useful sites and apps.

→ PayPal Honey If you download this browser extension on your computer, it will pop up when you’re viewing an item on Amazon, Walmart, Macy’s and many other shopping sites. Hover your cursor over the Honey icon and click on View Price History to bring up a chart showing the product’s price over time. Add it to PayPal Honey’s Droplist to receive price drop alerts by email. (joinhoney.com)

→ CamelCamelCamel The browser extension of this Amazon-only price tracker will pop up while you’re on the e-tailer’s site. Or you can paste the web address of an Amazon product page into the search bar on the tracker’s website. (camelcamelcamel.com)

→ Pricepulse This smartphone app also provides detailed price histories and price alerts solely for Amazon products. It uses artificial intelligence to assess whether it’s a good time to buy, rating prices in a range from Amazing (good) to High (bad). (pricepulse.app)

→ Capital One Shopping Even if you’re not a Capital One customer, you can use this website to search for better prices on products and sign up for price alerts. Alternatively, you can install the tool on your desktop as a browser extension. (capitaloneshopping.com)

→ Google Shopping To get price alerts from this site, do a Google search for a product, then tap or click the Shopping tab. Scroll down until you see boxes with flag icons. Click on the product name to open a box, then click on View Product Details and look for a box reading Typical Prices Across the Web. Below that, for many items, is a Track Price feature for notifying you when the price drops. (shopping.google.com)

→ Flipp For when you’re shopping in stores, this mobile app will help you keep on top of sale prices in printed circulars. Set up a watch list on the app, and it will automatically round up flyer deals on those items as they pop up. (app.flipp.com)