It's one part of shopping that both retailers and their customers loathe: the checkout line.

Even customers who like to shop just want to get out the door. And it's not lost on retailers that a good portion of their labor costs go to workers who perform robotic tasks.

Meijer last month joined chains like Sam's Club and Macy's that let shoppers ring up purchases on their smartphones. To retailers, the cashierless future is not such a leap. No one thinks twice about using ATMs or pumping their own gas. Eventually, retail experts say, streamlined serve-yourself shopping could feel just as natural. But "it's going to be a bumpy road," said Robert Moraca, vice president for loss prevention at the National Retail Federation.

By now, self-checkout kiosks are a staple in grocery and drugstores. But it's become a bigger focus for a wider range of retailers in the past couple years, said Read Hayes, director of the Loss Prevention Research Council at the University of Florida.

Customers are watching closely. When Chicago's first Amazon Go store opened last fall, customers eager to try what Amazon calls "just walk out shopping" stretched down the block.

Customers at Meijer's 23 stores in Chicago and northwest Indiana now can use a smartphone app to scan a bar code on items as they take them from the shelves. It still requires a stop at a self-checkout kiosk, but doesn't require pulling each item from the cart and makes it easier to search for coupons, said Thomas Dant, Meijer's Chicago-area market director.

Sam's Club and Macy's have similar tools.

At least temporarily, there's one more friction point for Meijer shoppers scanning purchases with their phones. An employee has to spot check their work.

It's a short-term measure while customers and employees get used to the new system, though Meijer said it will audit some purchases on an ongoing basis as a security measure.

Still, it highlights the challenge for retailers that want to give customers a seamless experience without making it too easy to walk away with items they haven't paid for, whether intentionally or by mistake.

It's true that some folks will "forget" to scan. Stores with self-checkout tend to see higher losses than those that send all shoppers to cashiers, and letting customers use their own devices is generally seen as riskier than requiring that they ring up their items at a kiosk because it's harder to monitor, Hayes said.