Working for Instacart — buying and delivering groceries to strangers — at first felt like Michaellita Fortier's childhood dream of starring on the speed-shopping TV show Supermarket Sweep.

"It was fun in the beginning," Fortier says. She felt like she was helping people in need while making as much as $16 or $20 per delivery.

But then the app inundated her with orders worth half that, $7 or $9 per delivery. For that money, she was expected to go to the store, shop, fill a cart and deliver an order, sometimes driving 10 or 15 miles.

"I thought, 'Now listen, this is less than minimum wage you want me to drive [for],' " Fortier says. After seven months of working for Instacart in west Michigan, she quit the gig in October.

Millions of Americans like Fortier have counted on platforms like Instacart, DoorDash, Uber or Lyft not just as a service but as a job. Quickly, they find themselves at the mercy of an algorithm — ever-changing pay structures, no assurance of a minimum wage, the smallest tweak of the app capable of upending their livelihoods.

In recent months, Instacart — the most popular grocery-delivery app — has faced a new wave of discontent and even a strike over pay. The company relies on more than 130,000 people to log on and help strangers with their groceries. As NPR first reported in May, the majority of them are women. All are independent contractors — and their grievances are extensive.

NPR spoke or exchanged messages with more than 40 current or former Instacart workers. At the heart of their frustration are the constant and murky changes to how the company assigns work and determines its value.

"It was extremely clear before how much you would be paid per item, how much you would be paid per order," says Liz Temkin, 63, who has worked for Instacart in Los Angeles for over four years. "And then they came up with a completely different what we call a black box algorithm."

Value of work

An Instacart representative declined to speak on the record but pointed out that in February, the company did commit to a guaranteed minimum pay: $7 to $10 per job.

This one job can vary from a quick grocery run to a big stock-up at Costco. Or it might mean multiple orders compiled together — delivering several payments to Instacart and one to the worker — though Instacart says its pay does account for the size, weight, distance and other factors of all the included orders.

"We used to get $10 to go to the store and 40 cents an item. Now, no matter how many items and how many units we get, they pay us $7. Maybe there's a tip; maybe there's not," says Mercedes Maltese, 42, who joined Instacart in 2017 in a suburb of Detroit after losing full- and part-time jobs in succession.

Maltese says her options are other minimum-wage jobs and with Instacart she can at least try to work as many hours as possible. However, Instacart also does not pay for the drive to the store — only from the store to the customer — and so Maltese spends much of her workday sitting in her car, unpaid, waiting for a decent order.

"I work ... seven days a week," she says, her voice swelling with tears. "I just have to work and live in my car like a homeless person, in some kind of hopes of scraping up some kind of money in order to pay my bills."

In interviews, Instacart CEO Apoorva Mehta often brings up the difficulty of constantly matching the supply and demand of grocery orders to workers. His company, expected to go public as soon as next year, is valued at nearly $8 billion. In May, Mehta told CNBC that Instacart is profitable on every delivery.

Like Uber in ride-hailing, Instacart grew nationwide thanks to algorithms assigning and putting value on work — and to a virtually unlimited pool of workers giving the app a try. In August, Instacart said its worker ranks almost tripled in one year.

Algorithmic control

"Algorithmic despotism" was the term that sociologists applied to Instacart in an August paper. The researchers found that the company stood out among competing apps for just how much it pushed workers to commit long hours and accept unattractive gigs.

"Instacart, we found in particular, really did control the labor process or control workers' experiences and limited their autonomy over their time and over the work they could do in a significant way," lead author Kathleen Griesbach of Columbia University told NPR.

The research, which received some funding from the United Food and Commercial Workers union, studied algorithmic control of food-delivery workers by surveying 995 people who gigged for DoorDash, Postmates, Grubhub and other apps.

Instacart dismissed the study, arguing that it relied on an outdated understanding of the app's workflow. A company spokesperson did not address the research on the record but pointed out that its contractors now can work more flexibly "on demand" — grabbing assignments whenever, as orders come in — in addition to signing up for the two-hour shifts featured in the study.

The majority of the 40-some workers who communicated with NPR said they worked both the old way and the new way — and either way, being faster, nicer or more experienced did not guarantee better pay.

The shift system has various rules for getting or even rejecting work. Many who rely on Instacart for full-time work scramble every week to sign up for enough hours.

"I live and die by my 9:20 Sunday morning alarm," says Teri Smith, 46, who has worked for Instacart in Arlington, Texas, for over a year. Once the hours are available at 9:30, "it's a frantic tap to get all the hours before they're all gone — and then by 9:31 everything is gone."

The new on-demand structure leaves many workers glued to their phone screens. They rush against each other to snatch up orders, with little time to thoroughly calculate whether shopping and delivering is worth time, gas and extra miles on their cars.

"Trying to quickly analyze and make an educated decision ... is next to impossible," says Cynthia Phillips, 58, who has been shopping for Instacart in Southern California for almost two years. Instacart "gets to pay less and less for services knowing that someone will end up taking the batch."

Opaque changes

Instacart workers' latest strike this month was over changes to tipping, which workers say is still a hefty chunk of their income. Instacart used to set a default tip of 10% per order, but now it's half that. Meanwhile, the tip is in addition to a delivery fee and a service fee that both go to Instacart.

The subject of tipping is particularly sensitive for Instacart workers. In February, the company had to refund workers after they noticed that Instacart was using their tips to subsidize the minimum payment it promised.

Right after the strike, Instacart made yet another change: It stopped paying the $3 quality bonus that rewarded each five-star review from a customer. Workers point out that for many of them, this bonus had already been a downgrade from Instacart's previous scheme, which rewarded the best-rated workers with $100 for the week.

Strikers have accused Instacart of retaliation. The company told NPR that the timing was coincidental and that the $3 quality bonus had been a test that did not meaningfully improve the quality of work.

"I could usually count on getting that bonus about 30% of my time. [That's] 10 orders a week, so now I'm $30 down," says Smith, who picked up Instacart as her primary income in August 2018 after her family struggled to run a toy store they'd opened in a Dallas suburb.

In a matter of a year, Smith says, she has noticed Instacart's pay declining for each order: "I was making the same amount of money in 40 hours that I'm making in 50 now," she says. Asked whether she makes minimum wage, Smith says: "Oh, heck no." Not after expenses, she says, such as gas for her 12-year-old car.

Grocery delivery "is a service that's definitely needed for people ... and I love doing it," she says. "The only thing I'm asking for is to be paid fairly."