Thousands of Amazon Flex delivery drivers filed arbitration claims on Tuesday, alleging that their classification as independent contractors has resulted in unpaid wages, and the loss of overtime pay and expense reimbursements.

Approximately 15,860 Amazon Flex drivers across California, Illinois and Massachusetts submitted the claims with the American Arbitration Association, on top of an additional 453 similar cases that are already being litigated.

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The claims, collected over a span of four years by attorneys Joseph Sellers and Steven Tindall, contend that the drivers should be classified as Amazon employees instead of independent contractors.

Amazon Flex is the tech titan’s “gig economy” package delivery service, in which drivers can use a mobile app to sign up for shifts to deliver orders via the Prime Now and Amazon Fresh programs in their own vehicles.

Most Amazon Flex drivers earn between $18 and $25 per hour, although actual earnings are dependent on factors including location, tips received and length of time to complete deliveries.

One such argument for employee classification came from a driver that indicated that Amazon does not provide Flex drivers with 10-minute rest breaks when their shift lasts longer than 3.5 hours. Another claim said Amazon does not provide 30-minute meal breaks for drivers who work more than five hours a day, whereas another claim alleged that Amazon fails to provide itemized wage statements required by California law.

Additionally, if a Flex driver books a three-hour block on the app, they only get paid for three hours, even if deliveries come in that take more time, one claim alleged.

“The vast majority of Amazon Flex delivery partners finish their delivery blocks early,” an Amazon spokesperson told Sourcing Journal. “Amazon Flex delivery partners have the freedom to take breaks, stop to pick up their kids, run errands and work however they want.”

In a statement, Amazon said Flex “gives individuals the opportunity to set their own schedule and be their own boss, while earning competitive pay.”

All of the claims 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 definition and rights of a contract worker versus an employee has been up for debate in laws such as California’s Prop 22, which allows ride hailing and delivery drivers to be treated as independent contractors with some added benefits, including a minimum earnings guarantee.