By my estimates AMZN, canalized their margins for volume.
The postulates behind this tactic is to capture the "consumer engine".
Eventually driving your competitors to either to drop or flop your categories.
Tone flaw in this tactic is the supply chain. As orders for goods increase inventory prices increase thus volume sales slow, even with on time inventory systems. Also, competitors such as AAPL GOOG WMT, have deep pockets and there no reason to believe that these savvy market-watchers will not do the same.
AMZN sales are strong by some measures, but the cost of goods sold are rising leaving AZMN on the brink of financially insecurity. And so there a 3 billion debt.
Even if the "fiscal cliff," is resolved, those tax hikes are likely to cause spending cuts by consumers.