Amazon doubled the days of its annual Prime sale for 2025, making the sale a four-day event, from July 8 to 11. And so far, it seems to be paying off.

CNBC is reporting that on Tuesday, the first day of Amazon's four-day Prime sale, U.S. online sales jumped 9.9% year over year to $7.9 billion, according to Adobe Analytics.

It was the "single biggest e-commerce day so far this year," Adobe told the outlet.

Related:2 Simple Strategies to Save More on Prime Day 2025

But not all vendors are seeing increased sales. Bloomberg reports that Momentum Commerce, which has about $7 billion in annual sales on Amazon, said its first-day sales were down 41% compared to last year. The company manages online sales for 50 brands, includingBeats,Crocs, and Therabody.

John Shea, the founder and chief executive officer of Momentum, blamed the lack of urgency, noting that a sale with more days means time for "treasure hunting" rather than impulse buying.

"It all hinges on this four-day strategy being a success," Shea said. "It's a wildly unpredictable and uncertain year."

Still, Shea noted that there could be a last-minute urgency push. His company still expects a 9.1% sales increase compared to last year if people shop more on the final two days, he said.

Related:Amazon Is Expanding Same-Day Delivery to Thousands of Small Towns and Rural Areas

In an interview with Bloomberg Television on Wednesday, the head of Amazon Prime, Jamil Ghani, said so far, shoppers are buying "everyday essentials," and noted specifically that teeth-whitening strips are the fastest-growing category on Amazon.

Ghani added that it is still "very early" to be talking about overall sales numbers.

CNBC noted that Adobe expects total four-day sales for the Prime Day event to be "equivalent to two Black Fridays," around $23.8 billion.