Dealing stores a hard hand

When Grenon found it, she aimed her phone’s camera at the bar code and waited to see how its $169.99 price matched up using an app called RedLaser. Best Buy’s price was good, the same as a nearby Toys “R’’ Us, but $3 more than Walmart.com. She bought it anyway because, she said, instant gratification was worth the extra cost.

“I want to make sure I’m getting the best deal possible, and technology is allowing that,’’ said Grenon. “I don’t have buyer’s remorse.’’

Grenon, 38, is at the leading edge of a retail revolution that is changing the way people shop and the way traditional brick-and-mortar stores sell goods. With the rise of smartphones and dozens of sophisticated shopping apps, consumers can instantly find out who - online or at nearby stores - has the best price on everything from toasters to flatscreen TVs. And if they don’t like the price in the store they’re in - even after browsing for hours, trying on outfits, quizzing salespeople, or testing out gadgets - they can instantly buy items online with a few taps on their smartphones.

While traditional merchants have long complained that online sellers like Amazon have an unfair advantage because they don’t always have to charge state sales tax and don’t have the same overhead costs, the rise of the smartphone-armed shopper is bringing more retail heartburn to storefronts across the country.

That angst was clear when Amazon.com Inc. offered users of its Price Check app a 5 percent discount last Saturday when they used it to comparison shop in physical stores. Condemnation was even heard from Congress.

“Amazon’s promotion - paying consumers to visit small businesses and leave empty-handed - is an attack on Main Street businesses that employ workers in our communities,’’ said Republican Senator Olympia Snowe of Maine. “Incentivizing consumers to spy on local shops is a bridge too far.’’

But the Amazon app is just one of many programs, which are mostly free, that take the guesswork out of figuring out where to find cheap prices. In addition to Price Check, there’s ShopAdvisor and Google Shopper, to name a few, that have become available in recent years. Most get their data by looking at prices on retailers’ own websites.

“The ability to check prices on your mobile phone when you’re in a physical retail store is changing the way people shop,’’ said Sam Hall, director of Amazon Mobile, in a statement.

Among the 38 percent of American adults who use some kind of app on their cellphone, 46 percent have downloaded one to help with shopping, according to the Pew Research Center. The rate of mobile shopping is also rising fast. On Cyber Monday, the day for online sales after Thanksgiving, 6.6 percent of online sales were done on mobile devices, compared with 2.3 percent last year, according to IBM Corp.

“You can shake your fingers at these Web companies all you want, but this is exactly what they are going to keep doing,’’ said Andrew McAfee, a digital business expert at Massachusetts Institute of Technology’s Sloan School of Management. “You don’t have to be any great visionary to see what opportunities that [smartphones] bring for comparison shopping.’’

Brick-and-mortar stores are trying to do their part to win over the mobile shopper. For example, Simon Property Group, which manages malls such as South Shore Plaza and Liberty Tree Mall, has teamed up with Shopkick, an app that gives rewards for simply walking into a physical store.

Others like Harvard Book Store owner Jeff Mayersohn trade on guilt. “We have a sign that says, ‘Find it here, buy it here, keep us here,’ ’’ said Mayersohn. “We want to impress on people that we feel pretty strong that they support the businesses where they are getting this information.’’

Rick Henry, owner of local toy store chain Stellabella Toys, said competition with online merchants should take place on a more level playing field. That’s why he supports pending congressional legislation to allow states to collect taxes from online sales. “Brick-and-mortar retailers are obviously paying more expenses to have staffs and maintain stores in shopping districts,’’ he said.

But that may still not be enough as new technologies can bring price transparency to every smartphone.

“Retailers are going to have to figure out how to compete on something else like customer service or experience,’’ said Blake Scholl, chief executive of Kima Labs, a San Francisco company that offers a technology that speeds up smartphone purchases.

Sandra Petersen, 50, of East Boston, uses ShopAdvisor, which was developed by Maynard start-up Evoqu Inc. The app lets users set up lists of products they want but only at a certain price. When those items hit their price sweet spot, they get an e-mail.

“Now that I have the iPhone 4, I’m scanning things when I’m at the store,’’ she said.

She was recently at a BJ’s Wholesale Club and scanned a Keurig coffee maker. According to ShopAdvisor, the BJ’s $129 price was the best around, but she wanted it for $120. She’ll wait.

“It’s sort of like a game,’’ she said. “Let’s see where we can find a cheaper price.’’

Michael B. Farrell can be reached at michael.farrell@globe.com.