Even after two decades of torrid growth, the rise of e-commerce is still in its early innings. Online sales comprise less than 10% of retail sales, with research firm eMarketer estimating that retail e-commerce sales will increase to more than $4 trillion in 2020, up from about $1.5 trillion in 2015. With millions more people gaining access to the Internet every year, e-commerce appears set to grow at a rapid pace for at least another decade, and likely much longer.