The Globe and Mail reports in its Thursday edition that in September, more than 40 strangers logged in to Instacart, the grocery-shopping app, to buy eggs and test a hypothesis. A New York Times item says connected by videoconference, they simultaneously selected the same store -- a Safeway in Washington -- and the same brand of eggs. The prices were surprising: $3.99 for two shoppers; $4.59 or $4.69 for others (all figures U.S.). The shoppers were volunteers, participating in a study organized by the Groundwork Collaborative, a progressive policy group, and Consumer Reports, a non-profit consumer publication. On item after item, volunteers found significant differences. In a Target in North Canton, Ohio, some shoppers were charged $3.59 for a jar of Skippy peanut butter that others could get for $2.99. The Groundwork study found no evidence Instacart was basing different prices on customers' individual characteristics like income, ZIP code or shopping history. But there is little doubt that Instacart and other on-line sellers have the ability to do so. Companies including Delta Air Lines, Amazon and Home Depot have been accused of experimenting with such personalized pricing, only to retreat after consumer backlash.