Amazon.com (Nasdaq: AMZN) has seen its stock soar more than 2,000 percent over the past decade and more than 60 percent during the past year. From its thriving e-commerce business to its ridiculously profitable Amazon Web Services cloud-computing segment, along with its acquisitions of Whole Foods Market and video-doorbell leader Ring, Amazon is truly firing on all cylinders. (Last year, AWS contributed 10 percent of Amazon’s revenue – and some believe the business could triple over the next five years.)

Amazon’s revenue last quarter skyrocketed 39 percent year over year (on a constant-currency basis) to a staggering $51 billion, with operating income jumping 92 percent. Founder and CEO Jeff Bezos has confirmed the Amazon Prime service sports more than 100 million members. Those members not only enjoy the fast, free shipping on millions of items for which Prime is best known, but also have access to Prime Music, Prime Instant Video, discounts at Whole Foods and other little-known benefits. In fact, Amazon recently used those benefits to justify increasing the price of Prime by $20, to $119 per year, marking its first price increase in four years.

With significant and accelerating growth drivers, Amazon.com is well worth considering as a long-term investment. (The Motley Fool owns shares of and has recommended Amazon.com. John Mackey, CEO of Whole Foods Market, an Amazon subsidiary, is a member of The Motley Fool’s board of directors.)

Ask the Fool

Q: When one of my stock holdings doubles in value, would it be smart to sell it and buy a different stock that looks more undervalued? – D.B., St. Joseph, Michigan

A: Maybe – but maybe not. Focus more on how overvalued or undervalued you think the stock is and what you think of the company’s growth prospects than on how much it has grown.

Do you believe it still has a lot of room to grow? If so, consider hanging on to all or some of your shares. The stocks of many great companies have doubled and doubled repeatedly over the years. On the other hand, if, after doing some research, you’re not so sure about it, you might sell some or all of your shares, thereby locking in at least some gains. If you don’t understand the company’s business very well, selling is smart.

Meanwhile, if the stock has grown so much that it now makes up a significant chunk of your portfolio’s total value, consider selling at least some shares so that you don’t have too many eggs in that one basket. Even seemingly terrific companies fall on hard times – you don’t want too much of your money tied up in one stock.

Q: What do brokerages charge to buy or sell a stock? – R.L., online

A: In the old days, each trade could cost several hundred dollars. Today, though, many major brokerages charge commissions of about $7 or less per trade. They typically offer other services, too, such as stock research, banking and/or financial planning.

Don’t just focus on commission costs, though, when seeking a brokerage – especially if you don’t trade often.

My dumbest investment

My dumbest investment experience was when I couldn’t place some trading orders I wanted to make because of problems with my brokerage. It required me to go into the office to sign a couple of forms. I got mad and transferred two of my accounts to two other well-known brokerages.

Unfortunately, the other brokerages caused me grief, too, as it took a long time until I got the options trading rights that I wanted. One brokerage’s online trading screen didn’t allow for larger text to accommodate my poor eyesight, and the other brokerage surprised me with lots of extra fees – while having a confusing and hard-to-use trading platform. I ended up moving those accounts back to my original brokerage. – J.B., Camano Island, Washington

The Fool responds: You’re right to move your account if you’re displeased with your brokerage and believe that a different one will serve you better. Most major brokerages get good marks on a host of factors.

Be sure you assess many factors before settling on which brokerage to use. Consider trading commissions, but also think about whether you want a brick-and-mortar location and about minimum investments and fees you’ll face, among other things. It can be a good idea to call each candidate’s customer service, too, to see how responsive and helpful it is. See if you can get a good look at how easy to use their online trading system seems to be, too.