The consumer has come to expect guaranteed overnight or 2 day shipping. When they don’t get that, they get really upset.
Amazon doesn’t want its customers to get upset, and it can’t trust its shipping partners because they don’t have the burden of the consumer sentiment. No one blames FedEx for their late package these days, they blame Amazon. Heck, you don’t have to look hard to find product reviews that only gripe with the shipping time or condition.
Amazon is going to vertically integrate shipping, no question. The market demands it, and Amazon will probably eventually ship more packages than any of its competitors. And, Bezos isn’t dumb, so he’ll build and sell this capacity to others.
Amazon is reaching further and further into the economy, and I don’t see much stopping them.