[moioci]

Would the law permit Amazon to discount the delivery fee more than 100%? ("We'll pay you 5% to allow us to deliver to you.")

[nolok]

No, in view of the law both (book and delivery) are considered as two different "products" and they are only allowed 5% on the delivery. They can set the delivery price to anything they want, but it was 0 before so it doesn't matter.