I think one as to be more careful here. At a first level the company plays no role, it is just the customer expecting that mistakes are handled differently depending on who benefits. Replace Amazon with a private person you are intending to buy a used car from, would you still expect that mistakes - say a miscommunication of the price or miscounting of the money - in your favour are handled differently than mistakes in the sellers favour? And we assume no bad faith here, if the seller intentionally raises the price once you have taken a day off and driven a significant distance to complete the trade in the hope that you will pay the higher price in order to not have wasted your time, that is an entirely different story. Personally I would probably not do it, but I think it would be perfectly fine to sue the seller to get the initially agreed on price.

Expecting Amazon to be a good sport only makes sense after you have decided that mistakes should not be handled differently depending on who benefits and that you are therefore not entitled to keep your amazing deal. Then you can start to appeal to being a good sport, or the benefits for the public perception of Amazon, or whatnot to explain why Amazon should honor the deal. But this also brings up question like why would you not be a good sport in case of a mistake in favour of Amazon? Note that I do not want to imply any answers here, I just want to point out which kinds of questions one might have to consider.