Not only did Amazon.com Inc.’s two-day Prime Day sales event drive an Internet Retailer-estimated $7.16 billion in online sales for Amazon—a 71% jump compared with the previous year’s event—it also led retailers that sell on the retail giant’s online marketplace to significantly increase their spending on ads that appear on Amazon. And, based on data from multiple vendors that help facilitate those ad buys, that money was well spent.

For example, 61% of retailers that spent money on Amazon’s ads during Prime Day last year and this year increased their spending, according to Seller Labs, a technology provider that sells software to help merchants sell on Amazon and bases its results on merchants that use its tools. On average, they spent 119% more than they did last year.

Digital marketing firm Kenshoo reported similar results among its retailer clients; Kenshoo clients that advertised on Prime Day last year and this year, on average, roughly doubled their spending year over year on Monday, the first day of the sales event. And merchants in some categories were even more aggressive. For example, toys and games brands spent 6.3 times more year over year, health and beauty brands spent 3.1 times more and computer and electronics brands spent 2.0 times more.

Looking at the trends in a different way, Kenshoo clients spent 3.8 times more on Monday than they have spent on a typical day this month. That spending has paid off as it helped advertisers generate 5.8 times more revenue than on an average day this month.

While Prime Day is a prime time for spending on Amazon ads, retailers are spending more on Amazon ads than they did a year ago. For example, retailers that work with digital marketing firm Merkle to advertise on Amazon boosted their spending on Amazon’s two main ad formats, Sponsored Products and Sponsored Brands, 19% and 77%, respectively, in the first quarter, according to Merkle’s “Digital Marketing Report Q1 2019.” That spending increase helped boost sales stemming from Sponsored Products 101% and from Sponsored Brands 202%.

Sponsored Product ads are keyword-targeted, cost-per-click ads that can either appear on the right-hand side or bottom of search results and product detail pages on desktop and mobile devices. Sponsored Brands are keyword-targeted, cost-per-click search ads that appear at the top of the first page of search results on desktop and mobile devices.

Amazon is No. 1 in the Internet Retailer 2019 Top 1000.