If you're going to buy stocks with the intent of holding them for the extremely long term, you don't want to invest in companies whose growth is slowing due to their products' obsolescence. Rather, you want to pick companies that are paving the way toward the future, and leading in the directions where their industries are shifting. 

In short, you should consider companies that are already experiencing momentum and have huge addressable markets to grow into.

PayPal Holdings (NASDAQ:PYPL) and Amazon.com (NASDAQ:AMZN) are two such companies. They're well entrenched in payments and online shopping -- two areas that are undergoing significant disruption right now. Both have proven their mettle by delivering more than a decade of stellar performance. But the beauty of these companies is that they're still growing like startups, and there's no sign that will come to an end anytime soon.

Read on to find out why you should buy these stocks and never sell them.

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Ride the digital payment revolution

I believe one of the surest trends investors can bet on is the global shift to mobile payments. It's a market that is expected to grow at a compound annual rate of 34% over the next several years, reaching $4.574 trillion by 2023, according to Allied Market Research.

PayPal's recent performance is consistent with that forecast. The company's revenue has more than doubled over the last five years, and accelerated to a 23% growth rate in the second quarter. Total payment volume has consistently grown at a more than 20% annual rate for over a decade.

PayPal is in an enviable position in digital payments. It boasts 244 million customer accounts, and that figure is increasing at a double-digit percentage rate. The growing list of merchants who accept PayPal at checkout is now at 19.5 million. All those users provide PayPal a powerful network effect advantage, which is why large banks, credit card brands, and other digital wallet providers such as Alphabet, Samsung Electronics, and Apple have partnered with the digital payments leader.   

PayPal also owns Venmo, a peer-to-peer payments app that's extremely popular with millennials, who are less habituated to traditional banking methods. 

What's more, success has not made PayPal's management complacent. The company just acquired iZettle, a business similar to Square in that it offers point-of-sale solutions and an array of software services for merchants in 12 markets around the world. It has also completed other acquisitions recently that shore up its offerings in areas like risk management, and enhance its payout capabilities in online marketplaces. 

PayPal stock is richly valued, but for good reason. PayPal has proven it can stand with the biggest financial institutions in the world. Its brand is ubiquitous, and its long-term opportunity is simply massive. That combination should lead to big gains for shareholders.

It's still day one at Amazon

Forget about Amazon's recent flirtation with the $1 trillion market cap barrier. Ignore that its share price has climbed roughly 500% in the last five years. The online retail giant is led by a visionary CEO who makes decisions with the big picture in mind. This requires shareholders, in turn, to have the same mindset.  

Amazon continues to blanket the country with its network of massive fulfillment centers, and has even started to open physical retail locations. Yet as big as it has become, there are still many markets around the world that it has yet to penetrate meaningfully. Even when thinking solely about what Amazon is trying to accomplish in the United States, it still has major growth opportunities in same-day delivery, grocery delivery, smart-home devices, and Prime Video. E-commerce sales still account for less than 10% of total domestic retail sales, just to give you an idea. 

And all of that leaves out the power of its cloud computing platform, Amazon Web Services, which is growing like crazy and generates most of the company's operating profit. 

The company has been building its online retail empire for more than two decades, yet its trailing-12-month sales still grew 39% in the second quarter. It's that type of performance that justifies CEO Jeff Bezos when he says, "It remains day one" at Amazon. This is definitely a stock to consider tucking away in your portfolio forever.

Investing in high-growth, high-P/E stocks

PayPal and Amazon are not what you would call "undervalued" stocks. They sport high P/E ratios, but that just speaks to their great track records of growth, and the market's expectations that they'll deliver more. Investors rarely get the opportunity to buy great companies at cheap prices. In my experience, the best way to invest in these types of stocks is simply to jump on board when you can, and if the share price drops later, buy more shares.

John Mackey, CEO of Whole Foods Market, an Amazon subsidiary, is a member of The Motley Fool's board of directors. Suzanne Frey, an executive at Alphabet, is a member of The Motley Fool's board of directors. John Ballard owns shares of PayPal Holdings. The Motley Fool owns shares of and recommends Alphabet (A shares), Alphabet (C shares), Amazon, Apple, PayPal Holdings, and Square. The Motley Fool has the following options: long January 2020 $150 calls on Apple, short January 2020 $155 calls on Apple, and short January 2019 $80 calls on Square. The Motley Fool has a disclosure policy.