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 to $25 per hour, but the entirety of that payment doesn’t always come 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.