If the latter, I agree; that was sort of the whole point of my comment. Amazon controls a large portion of the shipping market, therefore what we have here is closer to a monopsony than an unencumbered free market.

If the former, I mostly disagree. Obviously a perfectly idealized free market system is impossible in the real world: all idealized systems are. There's no such thing as a frictionless surface or a perfect sphere either. That doesn't mean however that we couldn't be a lot closer to that ideal than where we are now, or that idealized systems aren't useful tools for modeling the behavior of similar, non-idealized systems in the real world.