He is worried that Carrefour, which uses self-checkout at its convenience stores to stay open on Sundays, will do the same at its larger markets. “First it’s Sunday afternoons, then it will be 24 hours a day,” Mr. Roche said.

Cashierless supermarkets are gaining ground around the world. Reducing cash payments and checkout time have become major goals for retailers that want to make the buying experience faster and more attractive while cutting labor costs.

Amazon pushed the boundaries by opening Amazon Go in the United States, a minimart where customers can buy items without any human interaction. Tesco is testing purely cashierless stores in Britain. Stores in China are increasingly using so-called facial payments, which lets shoppers pay by looking into a camera, after they have linked a photo of their face to a bank account. The facial payment technology eliminates the need for a wallet or mobile app.

Groupe Casino has not gone that far. But it is getting closer. Last year, it opened a gastronomic store off the Champs-Élysées in Paris that enables shoppers to buy everything from flowers to foie gras by using an app to scan the products and pay. Interactive displays show data on a product’s nutrition, price and popularity. For shoppers who cannot find the cheese, a voice-activated information screen gives directions to the right aisle. There are about a dozen workers who help customers and stock shelves, but no actual cashiers.

Tech-savvy and time-pressed customers have flocked to such services. “We’re meeting the needs of our clients,” said Mr. Corrado, the marketing director. “If we don’t have to close, then everyone wins because people get convenience and our sales increase. That also benefits our employees.”

That is hardly believable to workers who fear that opening all night at small stores, let alone Sunday afternoons at the big supermarkets, is a slippery slope to full automation and lost jobs.

Saliha Guechaichia, 47, grew anxious when the Géant Casino opened that first Sunday afternoon without her and the other cashiers, many of them single mothers with children. She began working at Casino 30 years ago, earning a modest income that helps her and her family get by.