On the surface, it looks like Amazon.com (NASDAQ:AMZN) continues to keep more money from each sale than it did a year ago. But a deeper look shows that margins didn't really rise from 25.3% to 27.7% -- they fell from 5.7% to 5.6%. The reason is Prime, Amazon's free shipping and streaming subscription that helps the company grow sales but hurts its bottom line. Erin Miller sat down with contributor Travis Hoium to see why investors should be concerned about Amazon's Prime push and its continued margin slide. 

Erin Miller has no position in any stocks mentioned. Travis Hoium is short shares of Amazon.com. The Motley Fool recommends Amazon.com. The Motley Fool owns shares of Amazon.com. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.