At what size company would your ethics require you to reverse course and treat them the way you would like to be treated if you had made the mistake?

Whose mistakes is it ok to take advantage from? It's hard to draw a bright line to split the continuum that goes from, say, the neighborhood nonprofit shop that employs homeless and disabled people from the community, all the way to a ruthless global mega corporation such as Amazon, but that doesn't mean that one's ethics are crooked if it draws a distinction between cases where it's OK and cases where it's wrong.

Is it your contention that a neighborhood nonprofit cannot be run ruthlessly? Do you contend that all of Amazon‘s employees are unethical? what about union pension funds that invest in Amazon-are they unethical?

Neither of these things. I am certainly not arguing that Amazon is unethical. Only that it can be ethical for people to make different decisions about when to take advantage of someone else's mistake on a case-by-case basis, based on the circumstances involved, without being required to produce a bright-line test valid in all possible hypothetical situations, as you seemed to imply. I'm just of the opinion that it's important to accept nuance and common sense in ethics, even if it means that sometimes dilemmas will arise that cannot be resolved easily. I'm also rather convinced that no human moral system exists without such dilemmas anyways. But the existence of dilemmas does not mean that every case is a dilemma, and it can be ok to make different decisions in different actual cases without having an answer to every hypothetical case in between.

It's also worth digging into "the mistake" here, because it's very likely that no human made this mistake (and can be punished for it internally). Rather the larger system (machine or beast) that is Amazon created a pricing system that created this price. You can either take advantage of it or not. So the real question of applying the principle of reciprocity here is ... what is the entity making the mistake and being exploited?

Would you like to live in a world where Amazon creates systems that exploit the market but individual consumers feel moral responsibility to turn a blind eye to the opportunities created by the mistakes that that system makes because they project their own humanity onto the system and apply the golden rule?

I'll take this one. At the point where the loss would create a cash crunch that would materially affect any of the company's employees (e.g. layoffs, company goes out of business, etc.)

If the company is Amazon-sized you are taking profits away from Amazon shareholders due to a mistake in the algorithms that drive Amazon's profits, which is nothing if not fair.

It's worse than that. "Given", hell, they have whole teams of people lobbying and marketing and scheming to create such chances.

That’s just simply not true.

Corporations are owned by regular people, especially the larger they are. Their shares are in retirement accounts and college funds, etc. That must count for something?

Not only is this mistake immaterial to Amazon's bottom line in the first place, it is even less material to Amazon's stock price (which is almost entirely predicated on the future), and retirement accounts / college funds diversify enough that there is a lot of dilution happening here. I'm having a hard time picturing a retiree or future student as a victim...