Wal-Mart (NYSE: WMT) recently agreed to buy e-commerce start-up Jet.com for $3.3 billion in a pricey attempt to challenge Amazon.com (NASDAQ: AMZN). Jet.com was founded two years ago, and its site is just over a year old. The company initially sold products at break-even prices for a $50-per-year membership fee, but scrapped the model last October, getting rid of the membership fee in favor of generating revenue from commissions paid by retailers.