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Price comparison apps — which allow shoppers in physical stores to quickly find out if they can get a better price online — have been called offline retailers’ worst nightmare. And that was before Amazon.com’s newest promotion.

This Saturday, Amazon will give shoppers using its Price Check app an extra 5 percent off purchases, up to $5, when they use the app to check a price while shopping in a store.

Amazon is also trying a crowdsourced approach, recruiting shoppers to collect prices in offline stores so it can try to beat them. The company says that it already scours stores and ads for competitors’ prices, and that it will help if users share the prices they find.

Amazon is the latest of many online retailers, including Gilt and HSN, that are using mobile apps to poach shoppers from physical stores. The apps blur the line between offline and online shopping, taking away offline retailers’ biggest advantage: once shoppers have driven to the store, they have added incentive to make a purchase.

Other price comparison apps include Google Shopper, eBay’s RedLaser and ShopSavvy. Some retailers, like Best Buy, have fought back by refusing to include bar codes on the products in their stores. Others, like Wal-Mart, have created their own mobile apps for shopping in their stores.

“The ability to check prices on your mobile phone when you’re in a physical retail store is changing the way people shop,” Sam Hall, director of Amazon Mobile, said in a statement. “Price transparency means that you can save money on the products you want, and that’s a great thing for customers.”

The deal is the first that Amazon has offered only to mobile shoppers. They can receive the discount on up to three items in eligible categories, which include electronics, toys, music, sporting goods and DVDs.

The Price Check by Amazon app is for Android phones or iPhones, and is available for download from the app stores of Android, Apple and Amazon. Shoppers can compare prices by scanning a bar code, snapping a picture of a product, speaking its name into the phone or typing the name.