Two law firms spearheading the action said about 15,860 Amazon Flex drivers have submitted arbitration claims with the American Arbitration Association, where 453 similar cases are already being litigated.

Amazon’s Flex program, which was founded in 2015, signs up drivers to deliver packages with their own cars and a special app.

The company pitches the work as a flexible, part-time opportunity that allows people to earn extra income during the hours they choose. Most drivers earn $18-25 per hour, according to Amazon, though how much they get paid can depend on other factors, such as their location and how long it takes to complete deliveries.

The arbitration claims submitted Tuesday were made by drivers in California, Illinois and Massachusetts, all of which have rules that limit the amount of control companies can exert over independent contractors. The claims, collected over a span of four years by attorneys Joseph Sellers and Steven Tindall, maintain the drivers should be classified as Amazon employees instead of independent contractors, based on current laws in the three states.

That change would allow Flex drivers to collect unpaid wages because Amazon only pays them for a pre-determined number of hours regardless of how long it takes to complete deliveries, according to the lawyers. It would also allow Flex drivers to receive overtime pay if they work more than 40 hours a week and get reimbursements for work-related expenses, such as gas costs and vehicle wear and tear.

Gas and other vehicle costs are a “huge expense to our clients,” Tindall said during an interview. He also said one client represented in the claims worked 7-day weeks making deliveries for Amazon during a holiday period and never was paid overtime.

In a prepared statement, Amazon spokesperson Branden Baribeau touted the benefits of the Flex program, saying it gives “individuals the opportunity to set their own schedule and be their own boss, while earning competitive pay.”

“We hear from most of the Amazon Flex delivery partners that they love the flexibility of the program, and we’re proud of the work they do on behalf of customers every day,” Baribeau said.

Tindall and Sellers say they have so far succeeded in seven of the eight arbitration claims against Amazon they took to trial. The drivers they represented in those cases were awarded an average of $9,000 in damages.

Amazon’s business model for its driving workforce – made up of independent contractors and third-party businesses that allow the company to avoid unionization – faces scrutiny and challenges from different corners.

A bipartisan group of more than 30 U.S. senators sent Amazon CEO Andy Jassy a letter last week asking for more information on the company’s relationship with the thousands of independent businesses that make millions of deliveries each day as part of Amazon’s Delivery Service Partners program.

In March, the Wisconsin Supreme Court let stand a lower court ruling that declared Flex drivers to be employees – a decision that would allow them to be part of the state’s unemployment insurance system and entitled to jobless pay if they are laid off.

The Teamsters union, which is seeking to organize Amazon’s drivers, also filed a complaint at the National Labor Relations Board last year challenging how the company classifies some of its drivers.