Amazon Now Has Grocery Delivery in Brooklyn has expanded its grocery delivery service to Park Slope a well-heeled Brooklyn neighborhood. The online service for Brooklyn started on Friday, giving the No. 1 online retailer in the U.S. a foothold in one of the nation’s densest and wealthiest markets.

The program – AmazonFresh – offers next-day or same-day delivery on over 500,000 items including both fresh as well as frozen groceries will soon move into other Brooklyn neighborhoods.

This move is part of the slow build out of Amazon’s of its program, which targets retail’s largest sectors yet to be overtaken by commerce online. Amazon would not say if it would expand into Manhattan or other areas of metro New York City.

A spokesperson for Amazon said currently the company is offering its AmazonFresh in the area of Brooklyn and continues its methodical and thoughtful expansion.

Groceries have shown to be one of the most difficult sectors for companies in technology to manage and competition is staring Amazon in the face with established companies such as FreshDirect and startups that are fast growing such as Instacart.

However, success in Park Slope would help Amazon to win customer loyalty and increase sales, especially amongst the middle-class and wealthy families, said analysts.

Amongst the wealthiest Americans, the top 10% to 20% spend up to 4 times more for their food than the average family in America, according to consulting firm that focuses on technology in the retail sector.

The wealthiest are the best of shoppers so anybody attracting their business is taking the cream off the top of the market.

Amazon could use its grocery delivery service to experiment with a delivery service of its own said analysts.

Fresh was tested in Seattle by Amazon for five years prior to adding San Francisco and Los Angeles during 2013. The metro area of New York presents a big change in challenges logistically including a much higher density in population.

Fresh is going to be offered to Brooklyn-based Amazon Prime program members for free through the end of 2014.

Following that, it will cost $299 per year for the grocery delivery program.