Instacart was founded in 2012 by Apoorva Mehta, now 27, an engineer who spent two years working at Amazon.com. “I had this problem of never having groceries in my fridge and never having the motivation and energy to go to the store,” Mr. Mehta said. But he found most grocery services, including AmazonFresh, too cumbersome. They required him to submit his order many hours in advance of delivery, and they sometimes didn’t offer the merchandise he was used to getting at his local store.

What’s more, traditional grocery delivery services are costly to set up, requiring warehouses to store perishable food, a fleet of custom-painted trucks for delivery and a staff of full- or part-time workers to package and deliver the orders. “We were the clowns who invested in WebVan, and we are only just getting out of therapy from that,” Mr. Moritz said, referring to the first boom’s spectacular grocery flameout.

Instacart does not maintain warehouses or trucks. Instead, the service is assembled out of found parts — existing supermarkets, willing part-time workers and their cars. The model has many advantages. It creates vast selection for customers by allowing them to shop at many different stores, from large chains to specialty shops like San Francisco’s worker-owned, vegan-friendly Rainbow Co-Op. It allows for extremely quick delivery too, including an option that will deliver your groceries in under an hour.

Though he declined to provide specifics, Mr. Mehta also said the firm had turned a profit in certain markets. Instacart now operates in ten cities, including New York, San Francisco, Chicago, Boston, Austin, Seattle and Los Angeles, and plans to be in 17 by the end of the year. It has 50 full-time employees and more than 1,000 independent shoppers under contract, many of whom fall into two demographic groups: college students and middle-aged mothers looking for flexible work.

Despite its low-cost business model, Instacart isn’t cheap. The service charges a delivery fee of $3.99 for most orders, and it also makes money by marking up store prices. The amounts vary depending on the item, but I noticed Instacart’s prices coming in at about 20 percent more than I could find at my local store.