A triple on a stock is quite the return. On average, the S&P 500 returns about 10.5% annually. At that rate, the S&P 500 would triple in 11 years from today's price. However, I wouldn't be surprised if these two stocks could triple in half that time. Shopify (SHOP 4.90%) and MercadoLibre (MELI 1.96%) are two e-commerce powerhouses, each serving a different niche.

Both are well off their all-time highs and have fantastic growth prospects. Here's why you might consider adding these two to your watchlist.

Shopify

Launching an e-commerce site is an ambitious task. There's the website design, payment processing, inventory management, and general upkeep of running the website. That's where Shopify comes in. With its software, businesses of all sizes can utilize Shopify to run an e-commerce website for a relatively low monthly subscription fee and a slice of each transaction.

During the pandemic, Shopify had some great quarters when nearly every store rushed to set up an online presence. Now that this wave of store signup activity has passed, investors aren't as excited about Shopify's future. As a result, its stock trades below its pre-pandemic price point.

I'm less pessimistic as Shopify still posted solid results in the first quarter. Revenue increased by 22% year over year to $1.2 billion, driven by the merchant solutions segment's growth of 29%. This area is crucial as it is the revenue Shopify derives from each payment it processes and from other services like the Shopify Fulfillment Network.

Most of Shopify's future revenue expansion will likely come from its merchant solutions division. Its 29% growth (while gross merchandise volume only rose 16%) is excellent news for investors as it shows some of Shopify's other products are starting to catch on.

Still, Shopify is marginally profitable on an adjusted basis ($25.1 million), which is concerning. For Shopify to triple, it needs to continue driving the expansion of its add-ons like Shopify Payments and Shopify Capital. If Shopify can maintain a 20% revenue growth rate (not an easy task), its revenue will triple in six years.

And with a valuation that hasn't been this low since 2016, I think Shopify stock has an excellent chance to triple over the next few years -- and even if it doesn't, it will likely beat the market from here.

MercadoLibre

MercadoLibre is more directly involved with commerce. Its online marketplace, payments platform, consumer credit, and shopping logistics are the go-to solutions for all things e-commerce in South America. While most e-commerce companies have seen a pullback in growth, MercadoLibre is still expanding rapidly.

On the commerce side, MercadoLibre's revenue increased 44% year over year to $1.3 billion in the first quarter ended March 31. While this marks a slowdown from last year's 188% growth in Q1, it is still respectable. Its fintech revenue is what's most exciting, as that rose 113% to $971 million, primarily due to its credit-portfolio growth.

Another noteworthy sign is that MercadoLibre's take rate increased on both the commerce and fintech side during the quarter. That's how much MercadoLibre gleans from each transition it processes through both its channels. The commerce and fintech take rates rose from 15% to 16.7% and 3.19% to 3.84% year over year, respectively -- showing how MercadoLibre is finding more ways to generate revenue.

Despite this success, MercadoLibre remains seriously undervalued. Trading at 4.3 times sales, MercadoLibre is as cheap now from a price-to-sales (P/S) standpoint as it was during the depths of the Great Recession.

MELI PS Ratio Chart

MELI PS Ratio data by YCharts

An all-time low valuation for a company firing on all cylinders seems strange to me. But if the market is giving me one of the fastest-growing e-commerce stocks on sale, I will take it up on its offer.

MercadoLibre's typical valuation hovers around 12 times sales. With its current multiple of four, MercadoLibre only needs to return to its usual valuation to triple. However, the strength of its business execution and a vast market opportunity in South America make MercadoLibre's stock a coiled spring.

Both Shopify and MercadoLibre make great candidates for stocks that can triple, but if I had to choose one, it would be MercadoLibre. Its valuation is just too low for the growth it's exhibiting. Still, Shopify makes an excellent investment as well.