The article does not say that downloading it over your wi-fi connection will result in $2.58 in delivery fees. It says that the average delivery fee for the book was $2.58.
And that fee would be more reasonable if Amazon allowed authors to provide optimised versions of their book suitable for e-readers that don't have high resolution colour screens.
I don't particularly approve of their pricing, but to be fair the book could be downloaded many times over 3G in the future included in that initial delivery cost.
https://kdp.amazon.com/self-publishing/help?topicId=A29FL26O...
Amazon charge $0.15/MB and the book was approximately 18MB in size.