A company that makes home grocery deliveries is expanding to Houston this week, signaling growing confidence the concept will work in sprawling cities.

Instacart, founded two years ago in San Francisco, initially focused on densely populated areas like its home city, New York, Philadelphia and Boston. But Matt O'Connor, who oversees Instacart launches, says the company has since found success in places like Los Angeles, Atlanta and Austin, where residents are more spread out and more likely to depend on their cars.

"We were concerned initially whether it would work in less dense cities," he said, adding that experience proved those concerns unfounded. " ... In fact, some of our least-dense cities have been the most successful."

Houston will be the 14th city served by Instacart, which partners with grocery chains and relies on a cadre of part-time delivery workers dispatched by a computer program and paid on commission.

The company initially is offering service for H-E-B and Whole Foods Market in select areas and will expand geographically, O'Connor said. Service charges range from $3.99 to $9.99, depending on the size of the order and how quickly the customer wants it delivered.

For an annual membership of $99, delivery fees are waived on orders of $35 or more.

More Information Instacart comes to town What it is: Home-delivery service for groceries launches in limited areas of Houston on Wednesday. Delivery areas: Inside the Loop, from downtown west, and outside the Loop, south of the Katy Freeway to Bellaire. Service area map: Precise boundaries at instacart.com/locationsParticipating groceries: H-E-B and Whole FoodsMarket. Service charge, under one hour: $5.99 for orders of $35 or more; $9.99 for smaller orders. Service charge, within two hours: $3.99 or $7.99, depending on size of order. Tipping: Encouraged but not required.

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"It's a great convenience when you can save the time you'd spend going to a grocery store and get the products you would have picked yourself," O'Connor said.

The service is popular among parents with young children, busy professionals, senior citizens and office managers, he said.

Instacart employs independent contractors who provide their own transportation. O'Connor said they often are students who like the flexible hours. He estimated there will be "hundreds" in Houston by year-end.

The company will likely have two full-time employees here to manage marketing, employee recruitment and customer service, O'Connor said.

Customers can place orders at instacart.com or through a mobile app from an Android or Apple device. They can order anything offered in the store, O'Connor said.

Instacart made its Texas debut in Austin in May, making some first-day deliveries on horseback. Instacart will make some of its first Houston deliveries in art cars. O'Connor, a Houston native, said customers who place an order Wednesday will get a free month of Instacart Express, in which the delivery fees are waived for bigger orders.

Retail giant Amazon.com also has begun grocery delivery in some areas, but it does not currently offer the service in Houston.

Locally run grocery delivery services include:

›› Rice Epicurean Market has offered the service for 14 years, vice president of marketing Phil Cohen said. The chain has one store, at 2020 Fountain View. Detailsat riceepicurean.com/rice-delivers.

›› Memorial Concierge offers delivery from several major grocery chains within 20 miles of the Memorial area and will soon expand to The Woodlands, owner GraceGulilat said. Details at memorialconcierge.com.

›› The Grocery Station offers delivery mostly to senior citizens in central Houston, Friendswood and Clear Lake areas. Details at thegrocerystation.com.