Customers have tended to linger, almost surprised that they can just leave the store without lining up, and returning frequently in the first two months since the first Amazon Go location opened in Seattle, said Amazon vice president Gianna Puerini.

“What we didn’t necessarily expect is how many people would stop at the end, on their first trip or two, and ask, ‘Is it really okay if I just leave?” Puerini said on Sunday at Shoptalk, a retail industry conference in Las Vegas.

In a new concept that threatens to upend grocery retailing, Amazon Go makes no use of cashiers, but instead relies on shoppers using smartphone apps to signal they’re in the store. Cameras and sensors inside then track what shoppers take from shelves and what they put back—and then charges them as they exit the door, using credit card information that is on file.

The technology threatens to push Amazon stores well ahead of rivals and upend how people shop at small grocery and convenience stores. Puerini said there are no plans to implement the technology in Whole Foods Market, the upscale grocery chain Amazon bought last year for $13 billion, despite Wall Street analysts’ speculation.

Other insights from the cashier-free store is that the top-selling item is chicken sandwiches—likely a result of regular business from nearby office workers. Meal kits and fresh fruit are also popular.

Though Amazon has not said whether and how much it may expand the concept, the technology is already proving to be something it could use more widely: the Amazon Go app gives it customer feedback in real time and helps dictate what the store should stock. That is becoming an imperative across retail because chains, whether selling sandwiches or cashmere sweaters, don’t want to miss out on sales, or conversely, find themselves with unsold merchandise they have to chuck or sell at clearance prices.

One example? Amazon Go heard from vegans who were annoyed about the cheese mixed into the salads, so it now offers cheese on the side, said Dilip Kumar, vice president of technology of Amazon Go and Amazon Books, said at Shoptalk.

There is a veritable arms race in grocery retail. Last week, Walmart announced it was expanding delivery across the country, while Target recently added same-day delivery to its arsenal.

While coy about how Amazon Go is faring, the executives said that shopping frequency and sales and other metrics would dictate any expansion plans. Tech news site Recode recently reported the e-commerce giant plans six more Amazon Go cashier-free stores this year.