I'm not defending the seller but maybe it took 10 minutes of his time to pack the book plus there's a cost with the packaging materials. Maybe he then had to drive, 5, 10, 15 or perhaps even 30 minutes to the closest post office, park his car stand in line, etc, etc, etc. No matter which way you look at it, there has to be at least 1/2 an hour of handling time which may well account for the $40 odd dollars over the actual postage fee. He possibly could have explained things a bit better.
I have a post office 3 klms away from me but be damned if I can get there & back in less than 30 minutes anytime of the day, any day. Especially bad on Thursdays.
K