Kiri, you're spot-on about price and promotion in particular -- this is what Amazon built its reputation on, so much so that I, as a constant Amazon consumer, don't even bother to check whether I'm getting the best price anymore. I figure I am, but I know for sure I'll get my item in two days as long as it's in Prime. That convenience has ultimately become more important to me and keeps winning me back.
Amazon has completely re-written the model for these four Ps by pretending it wasn't actually doing marketing, at least initially. For the first 15 years of their existence, they didn't even use the word "marketing" at corporate, eschewing it in favor of words like "merchandising" -- even when it was clear that what you were doing at your job was, in fact, marketing. (I saw this as a long-time Amazon "marketing" veteran.) Bezos believed that if you built something that was great, customers would advertise it for you, and he was right -- until a point. That point was Kindle.