Amazon isn’t the money-losing online bookstore it once was. Yesterday, the retail giant announced record quarterly and annual profits, with operating income increasing 86% year-over-year.

But when it comes to explaining this profit growth publicly, Amazon doesn’t disclose the whole picture. And that murkiness leaves unanswered the question of just how much profit comes from one of Amazon’s thornier business practices: Increasing the fees on the millions of independent sellers who are critical to stocking the shopping platform’s digital shelves.

First, let’s explain what part of Amazon’s profitability picture is clear: Amazon’s giant and growing AWS cloud computing business is a significant profit driver, accounting for 58% of Amazon’s operating profit in 2024—or $39.8 billion of the total $68.6 billion income from core business activities.

After that, things start to get cloudier. The remaining 42% of Amazon’s operating profits in 2024—$28.8 billion over the 12 months—come from Amazon’s non-AWS business. The problem is that that bucket encompasses many different, giant divisions, and Amazon lumps all of their profits together into two geographic sections: North America and international.

One key operation is the core e-commerce business of buying and selling inventory that Amazon is still mainly known for. Prime membership fees also support a massive revenue stream. Additionally, there’s Amazon’s fast-growing advertising business, which built a sturdy foundation through simple sponsored product listings in search results, and has since expanded into streaming video ads too.

And then there’s the fees Amazon charges its independent sellers for services like warehousing, shipping, and customer service. All told, Amazon brought in more than $150 billion in revenue via these fees in 2024. If that revenue stream was its own corporation, it would rank inside the Fortune 25.

Along the way, the cut Amazon takes from seller sales has also grown. Back in 2016, Amazon took a 1/3 cut of sales by merchants, in the form of fees and advertising, according to e-commerce research site Marketplace Pulse. But by 2022, it had surpassed 50%, according to the same firm. Then, last year, Amazon sellers—even longtime defenders of Amazon’s business practices—were outraged when the company announced a series of new, complex fees that many sellers said made doing business on Amazon less predictable and more expensive. (Amazon argued at the time the amount of fees many sellers would pay would instead decrease.) Fortune reported exclusively at the time that the Federal Trade Commission was probing the fee increases.

Back to the profit question: Where does the remaining $28.8 billion of 2024 profit come from? Amazon officials say that ads are a significant profit driver. CFO Brian Olsavsky this week called it “an important contributor to profitability”—and analysts estimate that the business line has even higher profit margins than AWS.

Beyond that, Amazon officials have talked up cost reductions in their warehousing and transportation operations, with CEO Andy Jassy remarking that cost reductions in moving each order from an Amazon warehouse to a customer’s door “has been a meaningful driver of our increased operating income.”

Part of those cost cuts have come from Amazon reorganizing its warehouse network into eight U.S. regions—each designed to store and ship goods to customers nearby rather than across the country. Amazon’s buildout of smaller, local delivery stations also has reduced transportation costs per order by shortening the distance from the warehouses to customer homes. Meanwhile, Amazon officials have cited increased automation, including the use of more futuristic warehouse robots, as helping become more efficient, and thus reducing costs and boosting profitability.

But there’s reason to believe Amazon’s seller fees may be meaningfully boosting the bottom line. A persistent question among top Amazon sellers is how much of the fees Amazon collects from them does it actually need to cover the associated costs and generate a fair profit, and how much is essentially an extra tax that Amazon can charge sellers because of its position as the largest e-commerce store in the U.S., by far? Amazon has never offered up this information, likely partly because of the scrutiny it would invite. Remember, the Federal Trade Commission’s current antitrust lawsuit against Amazon argues, partly, that the company has abused its power over these sellers. (Amazon has fought the claims, which are set to go to trial in October 2026).

Looking at Amazon’s revenue growth from seller fees over time may tell part of the story. Over the last six years, since Amazon began breaking out revenue from third-party seller fees, Amazon’s revenue from seller fees has grown more meaningful to the company’s overall top line. In 2019, seller fees represented 19% of Amazon’s revenue, excluding AWS sales. By last year though, seller fees accounted for more than 29% of Amazon’s non-AWS revenue. That’s a 53% increase.

Amazon’s e-commerce business has become more dependent on third party-sellers over that period, but not by that much. In 2019, the share of total Amazon merchandise sold by third-party sellers was around 53% versus 61% last year. That’s a relatively modest 15% share growth for third-party sellers.

This Fortune reporter ran the theory of Amazon seller fee growth being a quiet, but material contributor to Amazon’s overall profitability to well-known Wall Street analyst Mark Mahoney, now of Evercore ISI.

“I haven’t done the analysis you’re talking about, but makes sense,” Mahaney wrote in an email to Fortune. However, he said that growth in Amazon’s highly-profitable ad business, along with cost improvements in the warehousing and transportation areas, were likely bigger contributors to profits.

But Mahaney also believes this reporter’s hypothesis is reasonable, and that growing seller fees “may well be material” to the company’s record profits as well.

As for Amazon’s take on the seller-fee hypothesis, it’s not saying; spokesperson Angie Quennell declined to comment.