Wait, so what kinds of savings are actually available on Amazon in the first place?

I mean, I order tons of stuff. How exactly is Zinc saving me money? The books I order don't seem to vary in price, ever, and I've never seen promo codes for them either. I don't want them picking a third-party seller instead of Amazon, due to quality issues I've had in the past. They say they're not applying the affiliates discount to their own orders.

So what exactly are all these mysterious discounts?

(Also, if the orders are actually being placed through their own account, how is tracking handled? Returns? Customer service in general?)