In its ideal world, food-delivery service DoorDash would pay its contractors almost nothing–relying on customer tips to cover most of the fee quoted for a job. In the real world, conditions are usually not so ideal, as DoorDash reveals to Fast Company, disclosing the share of driver payments that come via tips from users. In about 15% of cases, customers leave no tip, and DoorDash pays the entire amount, the company tells us. (The default tip is usually set to 15% in the app.)

In the next roughly 45% of cases, customers contribute less than half the money required to meet the minimum quoted fee–called the “guaranteed amount”–leaving DoorDash to chip in the majority of the fee. (DoorDash calls the money it contributes to make up the difference a “pay boost.”)

That news may not do much to satisfy workers protesting the company’s unorthodox accounting of pay and tips–especially in light of concessions by grocery delivery service Instacart last week that set minimum payments from the company to drivers.

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The guaranteed amount for a DoorDash assignment, quoted in the driver app, comes from some combination of the company’s own payments and customer tips in 85% of cases. If a customer pays more, DoorDash pays less–as little as $1 (called “fixed base pay”), if that’s all is required to get pay up to the guaranteed amount. DoorDash also adds a dollar when tips meet or exceed the guaranteed amount for an assignment. Thus, drivers know the minimum they will make before accepting a job, but the final amount may be higher, depending on the size of the tip.

Protesting Instacart workers have accused the company of also using tips to subsidize pay. Instacart denied the practice, and a new minimum-pay structure introduced last week would seem to rule out the possibility. But DoorDash acknowledges applying tips to the guaranteed amount in its FAQ about the new payment system.

That’s not how tips or gratuities are supposed to work, says economist Sylvia Allegretto of the Institute for Research on Labor & Employment at the University of California, Berkeley. In a case like DoorDash’s, “Tips become part of a wage subsidy instead of becoming a gratuity,” she says. “I think most customers intend for them to be a gratuity.”