"These are like the good days that make you want to continue doing Amazon. But, you know, there's obviously bad days too," Montes said.

CNBC spent a day delivering with Flex driver Omar Montes in the San Francisco Bay Area, where he helped a friend deliver around 46 packages in 3½ hours, for $105.

Flex was launched in 2015, and now operates in about 50 U.S. cities. Anyone over 21 with a driver's license, auto insurance and at least a midsize sedan can sign up. After clearing a basic background check, drivers in areas with open spots can start picking up and delivering packages.

A big part of this fast shipping process is getting the package that last mile to each address. Amazon Flex uses on-demand contract drivers to help with this especially labor-intensive and expensive hand-delivery.

One-day shipping used to be a luxury. Now, Amazon is making it the norm for its 100 million Prime members. The faster speed is now available on more than 10 million products across the country with no minimum purchase required.

Last year, Amazon also brought on small-business partners and bought 20,000 Mercedes-Benz Sprinter vans for them to use for delivery. These two programs are part of Amazon's larger move to grow its own logistics network, relying less on other carriers like UPS, the U.S. Postal Service, and FedEx — which just announced it wouldn't renew its express U.S. contract with Amazon.

Amazon would not disclose how many drivers have signed up, or what percentage of its last-mile deliveries are made by Flex drivers compared with its shipping partners. But it did tell us the program is expanding.

"We've built out these small-businesses delivery service providers and we have Flex which is our on-demand crowdsource delivery. So we need all of that to meet the various types of delivery we do in each of our geographies and I think you're going to see expansion on all fronts there," said Dave Clark, senior vice president of worldwide operations at Amazon.

Flex drivers use the Flex app to sign up for a "block" — which ranges from three to six hours — then head to a fulfillment center, where they find out how many boxes they've been assigned to deliver in that time frame. Amazon advertises that drivers make $18 to $25 an hour. Flex drivers are responsible for their own vehicle costs like gas, tolls and maintenance.

Depending on how long a block actually takes to deliver, drivers said the jobs are not always worthwhile.

"If I spent three-hours-plus of my time, that's just doing the block. Then who knows how long it will take me to get back. And all I made was 70 bucks, and half of that might go to expenses like gas and like the bridge," Montes said.

But drivers in less-congested regions, including Arizona, Kentucky and New Jersey, told CNBC that they often finish deliveries early, making the pace relaxed and the hourly pay much higher, because you're paid for the full block no matter how long it takes you.

"You just might have just the one package in your three-hour block. That's done in 20 minutes. So that's rare but it happens, and it's just a nice little thing to happen to folks driving," said Quan Tsang, an Amazon Flex driver in Irvine, California.

Other drivers told CNBC they had safety concerns, from the Flex app encouraging distracted driving, to a lack of Amazon-branded clothing that leads to confrontations with confused customers.

Amazon told CNBC that "Safety is our number one priority. We communicate with delivery partners regarding safety topics, including safe loading practices. The vast majority of drivers complete their routes safely in less than the allotted time."

"Amazon provides delivery partners with 24/7 on road support," the company said. "If delivery partners come across issues while making a delivery, we work with them directly on a resolution.

CNBC spoke to a total of 11 Flex drivers around the country to find out what it's really like to deliver for Amazon, and how it compares to other gig jobs like driving for Uber, Lyft, Postmates or Doordash. Watch the video for more.