In the ’90s, Webvan built several $35 million, 350,000-square-foot distribution centers. By contrast, Instacart, which offers one-hour grocery delivery in 12 cities, has just 70 employees in a small office in San Francisco, all of them engineers and administrators. They never touch the food — instead they contract with “personal shoppers.” “We literally don’t have any warehouses, we don’t have any trucks,” said Aditya Shah, Instacart’s general manager.

Of course, the couriers still need to be paid. “The complicated part is not getting customers, it’s getting the product to the customers,” said Paulo Lerner, Rewinery’s founder, who fled San Francisco for Brazil. “If they charge a lot, it loses the appeal. If they charge less, it has a lot of appeal, but at the same time, they are running on losses.”

Instacart charges as little as $3.99 for grocery shopping and delivery. Yet Shah said its shoppers make about $20 an hour, plus tips, which makes profitability seem unlikely, even with the smartest algorithms routing shoppers through grocery stores and city streets. When I told him that, he sounded a lot like Borders back in Webvan’s heyday: “We’re really well funded, so that is not something we’re as worried about,” Shah said. “Growth is the most important factor.”

That growth-first philosophy is hardly unpopular in Silicon Valley, where a focus on expansion at the expense of profit has worked well for web businesses. Delivery start-ups are trying to bridge the digital and physical worlds — and that’s when things get expensive. “It’s a hard category — outside the Internet, where everything magically happens,” said Lerner, the Rewinery founder. “This is real work, hard work. ”

Josh Lerner (no relation to Paulo), who runs the entrepreneurial management unit at Harvard Business School, is similarly dubious. “Someone is paying for it, but it’s definitely something that seems to defy the laws of introductory economics,” he said.

The question comes down to how much people are willing to pay to be lazy. To economists, laziness isn’t necessarily a bad thing. To the sympathetic onlooker, these companies could be a step on the path to the world prophesied by John Maynard Keynes (and even “The Jetsons”), in which technology advances to the point that chores are replaced by leisure time. But even this suggests a gloomy outcome: On-demand delivery could create a two-tier economy — the people who can afford to hire others to do their errands and the people who do them. That is, unless Amazon succeeds in automating grunt work out of existence. (It already has robots that pick items off shelves and pack them in boxes; it wants to have a fleet of delivery drones.)

Or it might be useful to listen to Fred Wilson, the co-founder of Union Square Ventures, who lost a lot of money on Kozmo. “I wish we knew the answers to these questions, but we don’t,” he told me. “That’s what’s kept us out of this market.” He still signs up for new delivery start-ups as a customer, though. After all, the worst case is that we’ll go back to doing the same thing I did when Rewinery went under — running out to the store.