At a guess: the author feels ripped off in the same way I feel ripped off when I buy an airline ticket and have to pay $25 to check a bag, $12 for a sandwich, etc. If amazon's fee is $4.50 they should say so instead of burying a cost almost equal to their quoted fee in fine print.
Edit: also, amazon grants themselves MFN status [1] at the authors' expense. So they exploit their market size to make sure authors can't make up for the kindle being nearly 100% more expensive than competitors.
By "price-match" we mean where we sell the Digital Book in one
or more of the Available Sales Territories at a price (net of
taxes) that is below the List Price to match a third party's sales
price for any digital or physical edition of the Digital Book, or
to match our sales price for any physical edition of the Digital
Book, in any one of the Available Sales Territories.
Amazon charging 30%, then another $2.50 is sketchy, why not just charge 50%, or at least avertise the price as a fixed $2.50 plus 30%? Questionable on Amazons part, without a doubt.
an even closer psychological analogy would be the ticketmaster fees layered atop the face price of a concert ticket. (how it actually works, apparently, is that ticketmaster provides the service of being the "bad guy" so that venues/artists can hike their prices without alienating their customers, but they invent a bunch of crappy itemisations of their fee, leaving people feeling even more ripped off because the charges are transparent bullshit)
Edit: also, amazon grants themselves MFN status [1] at the authors' expense. So they exploit their market size to make sure authors can't make up for the kindle being nearly 100% more expensive than competitors.
[1] https://kdp.amazon.com/self-publishing/help?topicId=A29FL26O...