flocked to the software platform during the pandemic. About 9 percent of U.S. online shopping sales took place on storefronts powered by Shopify as of October, according to research firm eMarketer. That was up from 6 percent the prior year and second only to Amazon’s share of 37 percent.

Harley Finkelstein, Shopify’s president, said Google and Shopify were developing new ways for merchants to sell through Google services, such as experiments to allow customers to buy items directly on YouTube and to display what products stores are carrying in Google Maps.

Mr. Ready walked a fine line when it came to Amazon, which is a big buyer of ads on Google, but he made it clear he believed Amazon’s dominance in e-commerce posed a threat to other merchants.

“Nobody wants to live in a world where there is only one place to buy something, and retailers don’t want to be dependent on gatekeepers,” he said in an interview.

Google said it had increased the number of sellers appearing in its results by 80 percent in 2020, with the most significant growth coming from small and midsize businesses. And existing retailers are listing more products.

Overstock.com, a seller of discount furniture and home bedding, said it had paid to list products on Google in the past. But now that listings are free, Overstock is adding low-margin products, too.

“When all shopping starts and stops at Amazon, that’s bad for the industry,” said Jonathan E. Johnson, Overstock’s chief executive. “It’s nice to have another 800-pound tech gorilla in this space.”

BACtrack, a maker of breathalyzers, has more than doubled its advertising spending on Amazon in the last two years because that is where the customers are, it said, while it has spent 6 percent less advertising its products on Google.

“It seems like more and more people are skipping Google and going straight to Amazon,” said Keith Nothacker, the chief executive of BACtrack.

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