When Amazon AMZN started Prime Day three years ago, it was viewed as little more than a sales gimmick, similar to what Alibaba BABA does for Singles Day on November 11. But fast forward to today and you get the depth of the strategic planning.The third quarter is typically a quiet one for the retailer, so it’s a time that Amazon could afford to experiment with. And what could be better for sales than an offer for all-you-can-eat access to video streaming, music, e-books, free two-day shipping and more?Of course, all good things come at a price, and Prime, as this service is called, comes for $99 a year after a month’s free trial, or $10.99 a month, or, if you're a student with a valid .edu email, at half price or $5.49 a month for the months that you use it (after a six-month trial). Or, if you only want Prime Video, it’s available at $8.99 a month. But that isn’t all. Amazon makes it a point to shower Prime members with other deals and steals to ensure that they stick with it.Prime is a way for Amazon to incentivize people to buy from Amazon, and indeed, once you’ve paid for a benefit like free two-day shipping (or quicker on occasion), you’re likely to make the most of it. So Amazon decided to create a promo to sell this loyalty program. That’s what Prime Day is all about.Amazon had all the time and resources to service extra sales in what was earlier a pre-holiday lull. Moreover, selling Prime memberships at this time was a good way to ensure stronger sales in the holiday season that followed.In the just-reported quarter, Amazon reported very strong sales that were attributed mostly to the spillover effect of Prime Day, particularly in international markets where penetration is lower. Whole Foods, which benefited from more aggressive pricing, cross selling private label products on Amazon and integrating Prime rewards with Whole Foods, contributed $1.28 billion in the partial quarter since the acquisition closed. Other things like the pull-in of the Diwali season in India and the Souq acquisition also contributed.No wonder that eMarketer recently estimated Amazon’s share of the U.S. ecommerce market at 44% in 2017 and its share of the total U.S. retail market at 4%. Amazon made 38% of ecommerce sales last year, so this is a huge gain, and evidence of the way it’s sales continue to gain momentum. eBay’s share of ecommerce was estimated at a mere 6.8%, Apple AAPL 3.6%, Walmart another 3.6%, followed by Best Buy, Macy’s, Wayfair and Costco.The bottom line: Prime is Amazon’s biggest competitive advantage and there’s a lot of momentum building around it. It’s the main reason that North America sales grew 35% and international sales grew 29% in the last quarter, the strongest growth in recent quarters. The retail business isn’t where Amazon makes its profits however and for all its revenue strength, operating profits in North America continues to decline while international continues to post losses. That’s because investment for expansion and market share continues at a blistering pace.AWS is the main profit center, where Amazon has outsmarted Microsoft MSFT, Alphabet’s GOOGL Google, International Business Machines IBM, Oracle ORCL and just about everybody else. That business reported operating profit growth of 38% in the last quarter.Amazon shares currently carry a Zacks Rank #4 (Sell), which could change following its stellar results. But you can instead see the complete list of today’s Zacks #1 Rank (Strong Buy) stocks here.Wall Street’s Next AmazonZacks EVP Kevin Matras believes this familiar stock has only just begun its climb to become one of the greatest investments of all time. It’s a once-in-a-generation opportunity to invest in pure genius.Click for details >>Want the latest recommendations from Zacks Investment Research? Today, you can download 7 Best Stocks for the Next 30 Days. Click to get this free report International Business Machines Corporation (IBM): Free Stock Analysis Report Amazon.com, Inc. (AMZN): Free Stock Analysis Report Alphabet Inc. (GOOGL): Free Stock Analysis Report Apple Inc. (AAPL): Free Stock Analysis Report Microsoft Corporation (MSFT): Free Stock Analysis Report Oracle Corporation (ORCL): Free Stock Analysis Report To read this article on Zacks.com click here. Zacks Investment Research