Who is that stranger in an SUV delivering packages in your neighborhood?

Amazon Prime members who have placed orders for same-day delivery recently may have noticed something odd about the people delivering the package.

They aren’t in uniform and they’re driving up in unmarked cars of all makes and models.

The explanation? They most likely drive for Amazon Flex, a delivery service similar to Uber or Lyft but for packages.

It works like this: a driver signs into the Flex app, where Amazon posts “blocks,” or a time window for which they have packages that need to be delivered. Amazon quotes the price it will pay for that time window. The driver claims the block and then arrives at the warehouse at the indicated time. They’re given a set of packages, which can range from a dozen to several dozen, and the app designs a route.

At each house, the driver is supposed to attempt to deliver the package to the homeowner. If that’s not possible, the driver leaves the package and snaps a photo to show Amazon the package was delivered. Then on to the next address. The drivers get paid the quoted amount whether they finish the deliveries early, on time or late.

“I usually finish my deliveries within 50 to 75 percent of my allotted time,” said Jesse, a 36-year-old Flex driver from Sacramento. He did not want his last name used because he didn’t want to jeopardize his employment with Amazon. At least seven drivers contacted for this story either did not respond or didn’t want to comment on the record.

Jesse said he’s got the deliveries down to a science. He began driving for Amazon about six months ago and it took him a couple of weeks to figure out his system for efficiently loading the packages into his car – starting in the backseat, then the passenger seat, then the driver-side trunk of his sedan and ending with the passenger-side trunk space.

He was driving for Uber and Lyft when someone in a rideshare-driver Facebook group recommended Flex. After he was hired, it took a few weeks for him to get a block.

“But once I did, basically I didn’t really go back to Uber,” he said, because he made about $1,500 more a month with Flex. The pay fluctuates from about $18 an hour to $25 an hour, but can go higher.

“Today, it’s $27, that’s the highest I’ve ever seen it,” he said on Wednesday, adding that it’s probably because the holidays are coming up.

He works a regular day job and drives for Flex at night and on the weekends so his wife can raise their daughter at home.

“It’s been transformational as far as my economic situation and my ability to provide for my family,” he said. He likes that the app tells you up front how much you will make that night rather than hoping someone will need a ride in his area.

Over the past few months, some media reports have surfaced about Amazon drivers being confused for porch pirates or doorbell-ditchers, which makes Jesse nervous.

“How many times have people seen me and thought I was stealing something?” he asked.

In other cases, the drivers were caught doing something wrong, like the recent south Sacramento homeowner who was reviewing his surveillance camera footage and discovered a driver delivering Amazon packages had pooped near his driveway. The video went viral across the nation. Notably, the driver pulled up in a U-Haul van rather than a specific delivery truck.

Amazon did not say which delivery service the driver was contracted with.

Amazon says Flex drivers are vetted through a multistate background check and a review of vehicle records.

The drivers are independent contractors, which doesn’t thrill traditional delivery drivers. The Teamsters union, which represents UPS drivers, feels that the contractor label is creating a lower tier of the delivery industry, said spokeswoman Kara Deniz.

Amazon and other e-commerce businesses are “experimenting with these alternative delivery models that we feel undermine the standards” the union has fought for, such as retirement security and benefits, she said.

“There’s plenty of work that’s keeping our UPS members busy,” she said. But “to look to the future of the industry and the direction it could be going in ... it appears that there are these efforts to sort of have a bottom tier, which is not really acceptable.”

Flex drivers deliver for Amazon.com, Prime Now, the AmazonFresh grocery service and Amazon Restaurants, a meal service. Amazon deliveries from its various services account for more than 20 percent but less than 50 percent of home package deliveries, said David Ross, a global transportation and logistics research director at Stifel, Nicolaus & Co.

Flex drivers probably aren’t making most of those deliveries, he said, because Amazon also contracts with the U.S. Postal Service, UPS, FedEx and independent contractors.

“(Flex) is a very limited part of the market,” he said.

Ellen Garrison: 916-321-1920, @EllenGarrison