There is a good article on Seeking Alpha that shows the impact of declining average rev per unit sold on amzn's profitability.
I thought about this from my own experience and think it shows one the problem that amzn is facing.
About 5 or 6 yrs ago, I bought a 46" flat screen tv on amzn for around $2,000. Now, you can get a 46" tv from $450 on up.
Think about the impact of this price drop on amzn's profitability. First gross margins on things like tv's have been squeezed over the past few years. Let's say that gm% went from 25% to 20%. Unit gross margin on the $2000 tv is $500 and unit gross margin on the new tv is $90.
Next, think about what has happened to shipping cost over that same period. At best, its flat (I think its probably up). I don't know how much it costs to ship a 46" tv, but it has to cost at least $50 (probably a lot more for next day delivery).
So, years ago, amzn would make around $450 on a 46" TV. Now they make $40.
This is one of many reasons why amzn won't be able to deliver the earnings that justify its current price.