Some ideas for Internet businesses are hard to explain. Think online advertising, where your personal profile - totally anonymous, we swear - is bought and sold in split-second, online auctions.
Some are easy to describe but fraught with complications. Think of the car services Uber and Lyft, whose battles with state officials highlight the risks of using a "sharing-economy" method - an app dispatching ordinary drivers in personal cars - to supplant an established taxi industry heavily regulated for safety and price.
Then there's Postmates, a concept simple enough to describe in a handful of words: "It's like Uber for stuff," as one pitchwoman put it. Or, as CEO and founder Bastian Lehmann prefers, "like a remote control for your city."
My city, Philadelphia, is the 11th market for a company that has drawn more than $23 million in venture funding and lots of buzz since its 2011 founding, even though Postmates inhabits an increasingly crowded niche.
Just Wednesday, Amazon announced that its version of near-instant gratification - "Get It Today" - is available in six more cities, including New York and Washington as well as Philadelphia. Instacart offers deliveries from half a dozen retailers, including wine and liquor from Pennsylvania's State Stores. Even Uber is getting into the game, with its UberRush courier service, though so far just in New York and Boston.
So how does Postmates fit in?
The San Francisco start-up isn't speeding you stuff it sells, or specializing in training "personal shoppers" to pick out your produce and other groceries, the niche defined by Instacart.
Lehmann says his aim is much broader. He wants Postmates to enable people to "hack the city" - to tap into an expanding list of restaurants and retailers, including wine and spirits stores, if possible, in order to satisfy a wide range of needs and whims.
"With Postmates," he says, "basically every store delivers in less than an hour."
Simple in concept doesn't mean simple in execution, of course. Delivering a test order Wednesday during Postmates' "soft launch," cyclist David Thompson demonstrated some small kinks along with the service.
I ordered a $4.95 vegetarian banh mi from QT Vietnamese Sandwich, a restaurant a few blocks from my office (and one that also offers delivery, apparently for a flat $6.99, via Dashed, another Postmates competitor). The sandwich arrived within 45 minutes, even after my first Postmate dropped the assignment.
Postmates charged $5 plus 9 percent in fees, but is waiving the $5 delivery charge - a price that can rise depending on distance traveled and demand - until Aug. 17.
I paid through the app, signing for the order after adding a tip on Thompson's smartphone - couriers can't take cash. Then he suggested why the first Postmate dropped the job: Thompson had to front the cash for the sandwich.
Lehmann says Postmates sees a special opportunity in smaller restaurants that don't take credit cards or offer their own delivery services.
"It's up to the Postmate to decide" whether to accept a cash order, Lehmann says. "If they do it, we refund the money within 24 hours, electronically."
Thompson, 23, said he was fine with the arrangement, in which he keeps 80 percent of the delivery fee plus the tip, but said he'd have to limit cash orders. He found the job on Craigslist, while awaiting work at a new West Philadelphia restaurant.
For Thursday's formal launch, Postmates is offering "free soft pretzels on demand," including delivery, from Miller's Twists - "limit one order per customer while supplies last," said a Postmates spokeswoman.
Lehmann says the busy competition is a spur to make Postmates better.
"At the end of the day, the best service for the customer wins. We're trying to build a service that customers love."