There's no one-size-fits-all way of investing. Some folks prefer the value approach, while others focus on dividends. There are those that think momentum is the way to go, while some swear by high-growth.

Even for investors that prefer a more conservative approach to investing, adding a few already successful growth companies to your portfolio can give your returns a noticeable boost. And just as there's no one single approach to investing, high-growth stocks come in all shapes and sizes. However, many of today's most successful companies have one thing in common: the growing importance of e-commerce.

Online sales currently account for about 10% of all retail worldwide, but that number is growing rapidly, as more and more merchants join the online sales revolution. Some of the front-runners in the field today could become the massive winners of tomorrow. But where's an investor to start?

To help narrow the field, we asked three Fool.com contributors to identify high-growth stocks that could still soar. Here's why they chose Bitauto Holdings (NYSE:BITA), Shopify (NYSE:SHOP), and Amazon (NASDAQ:AMZN).

Woman scattering $100 bills

Image source: Getty Images.

A one-stop online shop for cars

Leo Sun (Bitauto): Bitauto is an online services provider for Chinese automakers. Its website offers car shoppers data and prices for vehicles, and it facilitates direct sales between consumers and auto dealers. The company went public in 2010, and is backed by Tencent (NASDAQOTH:TCEHY) and JD.com (NASDAQ:JD).

Tencent integrates Bitauto's features into WeChat, the top mobile messaging app in China, while JD.com integrates Bitauto's platform into its own marketplace. Bitauto is also expanding into the used-car market via a partnership between its transaction services subsidiary, Yixin, and used-car transaction provider Yusheng.

Bitauto generates revenue from ads, site subscriptions, transaction services, and digital marketing solutions for automakers. Last quarter, its total transaction values more than doubled, its ad and subscription revenues rose 16%, and its digital marketing revenues grew 28%. Its total revenues rose 52% annually for the quarter, and analysts anticipate 23% growth for the full year.

Young woman using smartphone at a cafe

Image source: Getty Images.

However, Bitauto's non-GAAP (generally accepted accounting principles) net income also tumbled 76% annually last quarter due to higher marketing expenses and credit-loss provisions. That decline looks bleak, but Wall Street expects Bitauto's investments to pay off and generate 83% non-GAAP earnings growth this year. Based on that forecast, the stock looks surprisingly cheap at 16 times this year's earnings.

Bitauto is a risky stock, and it faces tough competition from Autohome, which operates on a similar business model. But if it plays its cards right, it could rally much higher as Chinese consumers shop for more cars online.

An e-commerce play with increasing potential

Chris Neiger (Shopify): Shopify has gained a lot of attention from investors lately, and for good reason. The e-commerce platform company helps merchants of all sizes sell their products online, and it has more than 600,000 businesses on its platform -- more than double the number just two years ago.

The company has also seen explosive revenue growth, with sales jumping 68% in the first quarter. Shopify is still very much in growth mode right now, so investors should know that the company isn't generating a profit. Increasing sales and more recurring revenue should eventually help change that, though. Shopify's management forecasts revenue of about $1 billion for the full year, which would be a 48% jump from last year. Additionally, the company is quickly building up a long list of larger customers, which spend more for its services and bring in 22% of the company's recurring sales.

The Shopify app, shown on a laptop, tablet, and smartphone

Image source: Shopify.

Shopify's stock is up more than 80% this year, but there's good reason to believe the company won't be slowing down anytime soon. That's because online sales represented just 9% of all retail sales in the U.S. last year, and by 2021 they will barely reach 14%. There's plenty of runway for the e-commerce market, and lots more opportunity for Shopify. As the e-commerce sales trend continues to grow, it should help boost Shopify's revenue and, eventually, its earnings -- and hopefully keep its share price ticking up.

A massive e-commerce opportunity -- and more

Danny Vena (Amazon): Amazon may not be the most original choice, but if current trends continue, the leading online seller is positioned to be among the biggest winners as the world increasingly turns to e-commerce for its shopping needs.

Recent reports suggest that Amazon could control nearly 50% of all digital sales in the U.S. by the end of 2018, after capturing an estimated 44% in 2017. The company's domestic online transactions this year are expected to exceed $258 billion, up 30% year over year.

A tractor trailer with the Amazon Prime logo on the truck.

Images source: Amazon.

Amazon is also working to export its U.S. success worldwide, and while those efforts are still in early days, there are signs the company is succeeding. In 2017, international revenue of $54.3 billion increased 23% year over year, and those sales appear to be accelerating. For the first quarter of 2018, international revenue of $14.9 billion jumped 34% compared to the prior-year quarter.

Investors with reservations about the company's ability to reproduce its U.S. e-commerce success elsewhere should remember that Amazon is also the worldwide leader in cloud computing. Amazon Web Services brought in $17 billion for the company last year, and with juicy margins of nearly 25%, it's much more profitable than Amazon's online sales. In fact, AWS represented nearly 10% of Amazon's revenue last year, and all of its operating income -- even subsidizing the company's international e-commerce expansion.

Amazon's stock performance has reflected the company's performance and massive worldwide opportunity, gaining 56% last year, and has already soared 55% so far in 2018. The company just completed its fourth annual Prime Day, its most lucrative shopping event of the year, expanding to more countries internationally than ever before. Amazon also reported adding "more new Prime members on July 16 than any previous day in Amazon history." Prime subscribers are the company's most prized customers, spending more than twice the amount of the average shopper.

With all that going for it -- and despite its already high growth -- I believe that Amazon stock could still soar.

John Mackey, CEO of Whole Foods Market, an Amazon subsidiary, is a member of The Motley Fool’s board of directors. Chris Neiger has no position in any of the stocks mentioned. Danny Vena owns shares of Amazon, JD.com, Shopify, and Tencent Holdings. Leo Sun owns shares of Amazon, JD.com, and Tencent Holdings. The Motley Fool owns shares of and recommends Amazon, JD.com, Shopify, and Tencent Holdings. The Motley Fool has a disclosure policy.