The U.S. Federal Trade Commission sent a letter to nine different companies on Wednesday demanding documents related to surveillance pricing, the practice of setting an individualized price for a given customer based on their data.

The companies include Delta, United, JetBlue, Lyft, Uber, DoorDash, Instacart, Target, and Walmart, all of which have been mentioned in news reports in recent years for potentially engaging in some form of surveillance pricing. However, most of the companies have denied the practice, and those caught doing it have insisted it was merely part of a pricing experiment .

The FTC’s document request says that each company has 45 days to respond, and it will probe not just how consumers are impacted but how workers are paid.…