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 attachment over 3g on my phone, essentially the same thing, I would be absolutely livid if I was charged $2.58.
Amazon's "delivery fee" is 15 cents/megabyte. By comparison, AT&T's most expensive pay-as-you-go data plan is $15 for 250MB, or 6 c/MB, less than half the price. Their cheapest plan is 1 cent/MB.
Amazon is known for having very reasonable data storage/transfer fees in their other services, so it's very clear to me that they're just taking a huge margin here.