But autonomous vehicles, like drones, aren't investible right now. They're too pie-in-the-sky to be on your radar as an Amazon investor. In the meantime, it makes sense for Amazon to continue to build on its Amazon Fresh service in additional markets, human drivers and all. There's no question that Amazon is a "castle in the sky" stock. It hasn't had a P/E ratio under 100 since 2011, and less than 2% of the firm's market cap is covered by cash on its balance sheet. Translation: AMZN isn't cheap by any stretch of the imagination. But momentum is still very much intact, and that means that the time is right for Amazon to make meaningful investments while costs of capital are low and its valuation is rich. Don't expect to see drones making dropoffs at your house by 2015 -- but same-day delivery still looks like a promising enhancement for AMZN. -- Written by Jonas Elmerraji in Baltimore.