As Amazon celebrated its 20th birthday Wednesday with a day of deals, it's worth noting that the online behemoth is often seen as catering to all shoppers—especially those looking for competitive prices and convenient shipping.
But one research group says disproportionately large number of Amazon's U.S. shoppers aren't those whom you would typically think of as needing discounts.
"Americans with money—and lots of money—are now shopping at Amazon and, as household income increases, so does Amazon's penetration of the upscale household segments," said a new report from the Shullman Research Center, which was based on a survey of 1,665 consumers.
Read More Prime Day isn't all about Wal-Mart
Seventy percent of adults with incomes of $250,000 per year have shopped on Amazon.com in the past 12 months, compared with 56 percent of consumers with household incomes of less than $75,000, the report said.
While Amazon may be a top destination for higher-income Americans, it's the top retailer for all income groups, edging out Wal-Mart, according to the data. The report said that over all income levels, 60 percent of Americans made a purchase at Amazon in the last 12 months, with only 58 percent reporting making a purchase at Wal-Mart.
And that rivalry may explain why Wal-Mart responded to Amazon's "Prime Day" discounts with its own sales event featuring a lower free-shipping threshold for shoppers.
Read More Caution signs for back-to-school
Wal-Mart's advertising also touted that its customers won't have to pay a fee to snag its best deals—a poke at the need for Amazon shoppers to be Prime members in order to benefit from the deals it's offering on Prime Day. (Amazon charges $99 per year for Prime membership, though there is a 30-day free trial.)
Amazon doesn't release statistics about its shoppers, and has never disclosed how many active shoppers it has or the total number of Prime members. The company did say that global Prime subscribers grew 53 percent in 2014 from 2013, but the base remains unknown.
However, the Shullman data found that the higher the income of the Amazon shoppers, the more frequently they shopped there.
Nearly three-quarters of the shoppers with a household income of $500,000 or more shop on the retail site once a month or more, the report said.
Consumer Intelligence Research Partners estimated that as of June, Amazon Prime had 44 million U.S. members. It estimated that the average Prime shopper spends $1,200 per year on the site compared with $700 annual for non-Prime members.
Shullman estimated that about one-third of Amazon's customers are Prime members. The group also estimates that among Amazon shoppers with incomes of $500,000 or more, 59 percent have a Prime membership.