According to the website, Amazon Flex is advertising for drivers to make $18–25 per hour delivering packages to users of Amazon Prime Now using their own cars and phones. They can sign up for to work in two, four, or eight hour shifts via a mobile app. Couriers can work as much or as little as they want. It’s a similar model to Postmates, which also allows couriers to deliver food from restaurants, groceries, and even retail items from stores. Even ride sharing giant Uber is intrigued by the courier business, testing deliveries of food.

The differentiating factor, for now, is that Amazon’s (AMZN) work force will be delivering packages of goods sold through its own site. And the service will only work for Prime members, who pay a fee to have access to a faster delivery on select items they buy through Amazon’s marketplace.

Amazon Prime costs $99 per year, and includes free two-day shipping on more than 20 million items on Amazon’s marketplace, as well as access to streamed songs, movies, and TV shows. Prime Now, which offers same-day delivery of items in 13 cities, is also included in Amazon Prime memberships.

Amazon says Flex is currently available in Seattle but will be rolled out soon in New York, Baltimore, Miami, Dallas, Austin, Chicago, Indianapolis, Atlanta, and Portland. Notably missing in that list is San Francisco, which is a competitive region for an on-demand app or service. As Fortune writer Kia Kokalitcheva wrote recently, many on-demand apps, including Uber and Shyp, have used San Francisco as their testbed because the city is small enough for startups to tackle and is filled with early adopters with plenty of cash.

For Amazon, Flex basically allows the company to control the shipping and delivery experience. With most Prime packages, Amazon has been relying on outside shipping companies and couriers for deliveries.

The drawback, however, is the ongoing backlash against the so-called “gig-economy,” which doesn’t necessarily include full-time benefits for contract workers. According to The Wall Street Journal, Amazon Flex workers will be contractors.

For more on Amazon’s on-demand strategy, watch this video:

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