Save couponAmount and productAmount in newly created Order

In order for the couponAmount and productAmount to be correctly stored
during th CartOrderTransformer::createOrderFromCart() process, we must
ensure that a correct amount is initialized in the Cart object when the
cart.onload event is triggered.
I wonder… if those concepts are orthogonal, why setting them in the same place?
After all, this class belongs to `Cart` component, and it should not know about the `Coupon` component.