Earlier quoted context omitted.
Hosting a static file in S3 you have to point people to manually find and download themselves is much cheaper because you don't have the built in captive audience, WhisperNet and everything else which comes with the Kindle ecosystem, so comparing the two only via a flat dollar cost is naive.
You are completely missing the point. They do charge a commission. The problematic part here is that this $2.58 is called "delivery fee". Im sure all the ecosystem costs a lot of money, and they are free to charge whatever they think the ecosystem is worth, but calling it "Delivery fee" seems disingenuous.
I think it is disingenuous to assume that $2.58 is some number pulled out of the air to rip the author off. The current meme right now is to assume all digital content delivery is essentially 'free', but this is untrue.