While spring is not necessarily when most people do their back-to-school shopping, the analysis tracked prices throughout the month of may May and the results highlight many of the ways Amazon’s pricing algorithm hides deals from consumers.

“I think most people think that Amazon has the best prices on products,” Adam Gauvin, vice president and head of product at Wikibuy, told Fortune. But he says taking the time to compare prices, even on small items, will often turn up a better deal.

Amazon begs to differ, although somewhat vaguely.

“All our customers are offered the same great low price on our back to school products,” an Amazon spokesperson told Fortune in an emailed statement, calling the Wikibuy research “flawed and misleading.” The person who spoke with Fortune declined to provide specific examples of the study’s errors on the record.

The way Wikibuy’s browser plug-in works, price comparisons cover all the typical costs associated with shopping online, said Gauvin, “including tax, shipping, and available coupon codes.”

For example, in this screenshot of Google Shopping results for a 24-pack of Crayola crayons today, Amazon initially appears to be the cheapest option. But as Wikibuy pointed out, tax and shipping costs for non-Prime members would make it more expensive than some of its competitors.

Wikibuy said it also sees some price variance depending on users’ geographic location and time of day. It found customers overpay by as much as 25% for items like graphing calculators in some states — as much as $30 in Rhode Island, or $20 in Texas and California. And customers who shop earlier in the day saw higher prices, the company said.

There’s an important and fine point to be made here.

Wikibuy’s study represents a month of shopper data from its users who purchased certain items on Amazon. So while they say they see a correlation between shopping in the morning and lower prices or shoppers in certain states having paid more for certain items, it isn’t conclusive proof that Amazon raises its prices in the evening or offers items at different prices in different states simultaneously. It’s simply a trend that’s evident in Wikibuy’s data.

Even now, at the dawn of the back-to-school deals season, the items Wikibuy flagged as having the biggest cost difference outside of Amazon — such as crayons, glue, and notebooks — were still priced higher on the e-commerce behemoth’s site than the third party retailers that sell on Amazon’s platform, according to cost comparison website Camelcamelcamel.com.

The news is timely for another reason: Just last week the Federal Trade Commission announced a probe into what it called Amazon’s “deceptive pricing” practices.

A 2016 ProPublican investigation into the company’s pricing algorithm found that it leads customers toward more expensive outcomes.

“You can sometimes find savings on products that you wouldn’t even think to price check. The small items that are $4, $5, $6 really add up to a larger basket,” Gauvin said. “You can find savings in places that you might not expect to look.”

He points to fixed-price items on eBay as one alternative retailer that many people don’t consider when shopping online.

There’s a very analog solution, too: Back to school shopping is one area where brick and mortar stores tend to win out over e-commerce. A study from Business Insider shows people would rather see school supplies in person before buying.

But with the majority of people turning to Amazon first when searching for products online, the opaque pricing is a concern for consumer advocates.

Shoppers purchasing back-to-school items online somewhere other than Amazon’s site would see savings hover between 8% and over 15% in any given week since this study was conducted according to Wikibuy’s price tracking, Gauvin said.

Some items saw notably fewer fluctuations in price, though: Amazon products.

“When we look at our data, the products that change prices the least on Amazon based on what our customers are seeing are actually Amazon products — like the Amazon Echo and the Amazon Tap,” he said.

Editors note: This story was updated on Thursday, July 27 to clarify the limitations of Wikibuy’s data analysis and the presence of Amazon Tap and Echo competitors on its platform.