Amazon customers who sign up for the e-commerce giant’s $99-a-year Prime program receive a variety of online benefits ranging from free shipping to free cloud storage.

Now Amazon is adding an offline benefit as well. For the past few months, clerks in Amazon’s three retail bookstore have been asking customers at checkout whether they are members of the Prime program, the web site Geekwire reported. Members get charged the same discounted prices for books that are available on Amazon’s website. Non-members, however, pay the full list price.

Books in the stores don’t carry price tags. Instead, customers have to use a scanner to see the price. The scanners now return two prices for each book, one for Prime members and one for non-members, Geekwire reported.

The policy is similar to Barnes & Noble’s $25-a-year membership plan. Members get free shipping for online orders and discounts of 10 to 40% off purchases in stores.

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Amazon’s stores are an experimental effort, designed in part to test what online retail strategies and tactics also might boost sales in physical stores. Differential pricing could prompt more consumers to sign up for Prime, which has been shown to make Amazon shoppers more loyal. Amazon has previously brought other pieces of its online strategy to the stores, such as posting a book’s online reviews and star ratings. The stores also serve as a showcase for Amazon’s hardware products such as the Kindle e-reader.

CEO Jeff Bezos has said his company will open more stores in an effort to learn more about how to succeed at offline retailing. Amazon has opened modest-sized bookstores in Seattle, San Diego and Portland with plans to open soon in Chicago and a Boston suburb. The company is also planning to open a chain of small grocery stores, The Wall Street Journal reported last month.

In typical fashion, Amazon hasn’t said much about how the stores are doing or how many it ultimately plans to open. Last week, Andy Jassy, who heads the company’s cloud unit, Amazon Web Services, repeated the company line that the stores are an experiment. “So far, we’re really pleased with the early results,” he said.

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The moves into physical retailing come more than 20 years after Bezos started the company as a simple bookselling website. Now, Amazon amzn is the largest online seller of books and many other goods. At the same time, Walmart wmt , the largest physical retailer in the country, boosted its online efforts this year by buying the e-commerce startup Jet.com for $3 billion.