It's worth expanding on this. Because I think it gets lost in the rhetoric, whether deliberately or by accident.
A profit-maximizing firm sets prices that maximize its expected profit. This seems tautological, but it's important and I think it's implications aren't appreciated.
If your prices are already beating the competition, and you already have a solid reputation for low prices, there is no incentive to continue lowering prices, even as you cut costs. This is why monopolies are considered "bad", because they eliminate incentives for firms to care at all about the welfare of consumers.
In undergrad economics classes, you might learn that prices are set equal to marginal cost (including opportunity cost). But that is only true under specific and relatively strong equilibrium conditions, which I don't believe apply to Amazon.