I was recently trying to offer a cart-level promotion with “Checkout by Amazon” and while looking through their Developer Guide, I became very confused. Their promotion section lead me to believe that I could offer such a discount, but it wasn’t clear at all. I tried googling it, and other research to no avail. 🙁
I finally read through the XSD which is not the most fun things to use to try and solve a particular problem. But the answer was there. To offer a cart level promotion, you simple need <CartPromotionId>DiscountName</CartPromotionId> in your <Cart></Cart> section. And then the same <Promotions> section that they have laid out in the documentation for item-level discounts.
I hope that this helps someone find the information faster than I did.
This answers the question: “How do I offer cart-level promotions in checkout by Amazon?“