Saving for retirement can be a challenging endeavor that takes years of discipline and patience. Still, the rewards can be more than worthwhile, as the financial freedom resulting from years of diligent saving can allow a fulfilling life during your golden years. 

Retirement plans can have several types of assets, including bonds, real estate, and stocks, to name a few of the biggest asset classes. More specifically, two stocks that can help you build retirement wealth are Costco (COST 0.33%) and Amazon (AMZN -0.68%).

Each business is excellent at what it does, and holding the two growth stocks together in your retirement portfolio provides diversification benefits. If consumers suddenly become turned off by shopping online, then Costco will likely benefit at the expense of Amazon. If the shift to online shopping accelerates and hurts sales at Costco, then Amazon is likely to benefit. 

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1. Costco 

Shopping in one of Costco's stores is so popular that over 60 million households now pay an annual fee to gain the privilege -- nearly a million more than in the previous quarter. The company has gained a reputation among its customers for offering famously competitive prices on a wide assortment of products and services.

Costco has two membership tiers, Gold and Executive, where guests annually pay $60 and $120, respectively. What's more, Costco has historically increased membership prices every five years. Impressively, Costco boasts an overall membership retention rate near 90%. 

Costco has 809 stores in operation, with only 250 of them outside of the U.S. and Puerto Rico. And Costco management is confident that there is an opportunity to expand its presence. Over the next three years, it plans on opening 71 net new locations. That should allow it to grow its member base and increase its buying power, further cementing its famously competitive prices.

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2. Amazon 

The core of Amazon's business is the 200 million paying Amazon Prime members. Unlike Costco, you can still shop on Amazon without a membership. However, an Amazon Prime membership comes with enough benefits to entice 200 million people to pay for the upgrade.

Its business model is different from Costco's in that it attracts shoppers who wish to order online. In fact, over the last six years, shopping online as a percentage of overall shopping worldwide has increased from 5.8% to 11%. That trend was only exacerbated during the coronavirus pandemic and is estimated to reach 19.2% in 2024. Some folks who increased their online shopping during lockdowns may not revert to old habits in the aftermath of the pandemic. 

Somewhat related to its e-commerce operations but also separate, Amazon's advertising revenue is growing rapidly as well, reaching $24.4 billion in the trailing 12 months. Indeed, in the last four quarters, advertising revenue has accelerated each quarter, growing 41%, 49%, 64%, and 73%, respectively. Clearly, businesses will pay to market to 200 million customers with their credit cards on file and who are one click away from making a purchase.

And supplementing the value Amazon is getting from its Prime members is its highly profitable cloud segment, Amazon Web Services (AWS). The service gives businesses the option to buy computing power from Amazon, rather than devoting large capital to building out their own networks. That gives businesses the benefit of paying for usage as they go, rather than making a risky upfront investment. As the market leader in the cloud segment, Amazon benefits over time as more business activities are moving online.

Investor takeaway

Holding these two stocks in a portfolio increases the likelihood you will build wealth for retirement. In a macroeconomic scenario that hurts one company, the other will likely benefit. Then when you boil down to the company level, each has a defensible moat that will keep competitors away. For those looking to build wealth for retirement, Costco and Amazon stock are good choices.