Yeah, the article is very poorly written. What they meant to say was that Amazon invests $11 in each customer, and the result is a doubling of purchases. I am sure Amazon makes more than $11 out of every $900 in sales, which makes this a good deal for them.
The amazon profit margin is something like 3%, which some people consider unsustainable; 3% over the god-knows-how-many billions that flow through them is excellent business. The potential sustainability problem comes from having no margin for error or non-trivial change in structural costs. I don't know enough about it to have an opinion.
Regardless, 3% of $400 == $12, so with these numbers, Amazon breaks even on a new $400/year client, and makes money on any client that spends more than that.
As a point of comparison, Tesco, one of the UK's largest companies, also has a 3% profit margin. They do take £1 in every 7 spent in the UK high street, so it obviously works quite nicely for them. So at volume, it is very sustainable.
