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.