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.
It's ridiculous to include the cost of the entire "Kindle ecosystem" with a fee called "delivery fee" - as other posts have mentioned, this is what commission is for. WhisperNet is another story - this is legitimately a part of the delivery - however, it's just 3G service that they leased from AT&T, and to assume it costs them even close to $2.58 per delivery is just ignorant. If I downloaded an 18MB e-mail attachmen…
Then be very careful you never accidentally do that when on international roaming (which 3G Kindles do have). You could easily end up being charged $250. And no, that is not an exaggeration (there have been much worse cases).