Amazon at times dips into the tips earned by contracted delivery drivers to cover their promised pay, a Times review of emails and receipts reveals.

Amazon guarantees third-party drivers for its Flex program a minimum of $18-25 per hour, but the entirety of that payment doesn’t always comes from the company. If Amazon’s contribution doesn’t reach the guaranteed wage, the e-commerce giant makes up the difference with tips from customers, according to documentation shared by five drivers.

In emails to drivers, Amazon acknowledges it can use “any supplemental earnings” to meet the promised minimum should the company’s own contribution fall short.

“We add any supplemental earnings required to meet our commitment that delivery partners earn $18-$25 per hour,” the company wrote in multiple emails reviewed by the Times.

Amazon insists drivers receive the entirety of their tips, but declined to answer questions from the Times about whether it uses those tips to help cover the drivers’ base pay.

“Our pay commitment to delivery partners has not changed since we launched the Amazon Flex program — delivery partners still earn $18-25 per hour, including 100 percent of tips — and on average drivers earn over $20/hour,” Amazon spokeswoman Amanda Ip wrote in a statement.

Drivers question why they aren’t getting 100 percent of tips on top of guaranteed pay.

“They just hide behind the fact that they guarantee $18 (an hour),” said driver Jeff Lee. “Sounds great but that $18 (an hour) guarantee could be all from customer tips while Amazon chips in zero.”

As Amazon has grown into one of the world’s biggest companies, it has relied heavily on contractors to help keep up with the pace of deliveries. These workers do not qualify for benefits offered to staff employees. Tipping is one way Amazon and other tech firms such as Uber and Lyft have moved to appease their contractor workforce.

But using tips to cover promised wages has proved controversial for delivery startups Instacart and DoorDash. On Tuesday, those companies came under fire after BuzzFeed News reported they used tips to fulfill some pay promises. Drivers have long suspected that Amazon uses their tips to hit promised wage targets, according to five former and current drivers who spoke on condition of anonymity for fear of reprisal.

It has been hard for drivers to prove — the company does not provide them a breakdown of their compensation beyond showing the total paid out, citing privacy concerns.

But two drivers tested their suspicions when assigned to deliver packages to their own homes.

Lee, who still delivers for Amazon, said he tipped himself $12 and change for a package he brought to his own residence.

“It was slow that day, and I had no orders to deliver, so I decided to place a one-hour order as a customer to see what the hell was going on with our tips as I knew I would be the next driver to deliver this one-hour (order) to my house,” Lee said. His base pay for the 1½-hour shift was supposed to be $27. Including tips, he received a bit more than $30 — suggesting Amazon contributed only $18.

“The problem most drivers have with Amazon is there is zero transparency about our pay,” Lee said.

The practice is legal in some states. Amazon would not say whether it dips into drivers’ tips, at least in California.