(NewsNation) — A new study shows you could be paying more than your neighbor for the same Instacart groceries, even when you shop at the same store.

The experiment, involving 437 shoppers across four cities, asked participants to add identical items to their Instacart carts during coordinated test windows and found that 74% of the items were offered to shoppers at more than one price.

For those items, the gap between the highest and lowest prices averaged 13% but in some cases reached 23%, according to the study conducted by Consumer Reports and the Groundwork Collaborative, a progressive policy group.

“Instacart is quietly running pricing experiments on millions of shoppers during the worst grocery affordability crisis in a generation,” Lindsay Owens, Groundwork Collaborative’s executive director, said in a statement.

In one instance, Instacart showed five different prices for the same carton of eggs at a Safeway in Washington, D.C. Some shoppers saw the eggs listed for $3.99, while others saw them priced as high as $4.79 despite adding them to their carts at the same time.

At a Target in North Canton, Ohio, Instacart listed saltine crackers at $4.69 for some shoppers, while offering the same item for $3.99 to others.

Overall, the Instacart basket totals varied by an average of about 7% for identical products added to carts at the same store at the same time. That difference could translate to roughly $1,200 a year in added grocery costs for a typical family of four, the study estimated.

“Americans shopping for groceries aren’t guinea pigs and shouldn’t have to pay an Instacart tax,” Owens said.

An Instacart spokesperson told NewsNation some retailers run short-term pricing “tests” on the platform to help them “learn what matters most to consumers” but said those tests are “never based on personal or behavioral characteristics” and are “completely randomized.”

A Target spokesperson said in an email that it’s not affiliated with Instacart and is not responsible for prices shown on the platform. Albertsons, which owns Safeway, did not respond to a request for comment.

The Groundwork study did not find evidence that Instacart was basing different prices on shoppers’ demographic traits like age, income, gender or race. However, the study’s authors said the company has the technical ability to do so.

Instacart, founded in 2012 and based in San Francisco, has become one of the largest grocery delivery and pickup service platforms in the country.

Algorithms can quietly change what you pay

The study’s findings come at a time when more companies are experimenting with algorithmic pricing tools that can adjust or test prices in ways shoppers may not notice.

Dynamic pricing has long been used by airlines, hotels and ride-hailing apps to adjust what shoppers pay based on timing, demand and other variables. But as algorithms become more advanced, both the possibilities — and the concerns about overreach — have grown.

Instacart, for example, advertises an AI-powered price optimization tool called Eversight on its website. The software promises to “unlock revenue growth” through “automated, continuous testing” of prices directly with shoppers.

“A small subset of our retail partners – 10 U.S. retail partners that already choose to apply markups – use Instacart’s Eversight technology to run limited online pricing tests,” Instacart said in a blog post Tuesday.

The company said the tests can result in some shoppers seeing slightly lower prices on staples like milk or bread and slightly higher prices on specialty snacks or craft beverages.

Instacart maintains that the tests do not constitute dynamic pricing and says prices “never change in real-time, including in response to supply and demand.”

The groups behind the latest study want the Federal Trade Commission and state attorneys general to investigate Instacart’s pricing tactics.

“This investigation shows how it’s becoming more common for companies to use hidden algorithmic pricing techniques without telling the customer,” Justin Brookman, director of tech policy at Consumer Reports, said in a statement.