I dont think its unwarranted at all. Dont rip off buyers by charging excessing handling fees.   Im glad the feedback is there as it lets buyers know up front you charge too much.   I have no problem paying a small handling fee but thats excessive.   If it was because of the items location why not of put that in the listing?   Good on the buyer for being factual

I assume kustom sent the book in padded bag. And thickness of the book and padded bag was just under 2cm thickness or Australia post worker assume it was under. That's why it went as $1.80 large letter rate.

Some postal workers make it as parcel rate if the side of the packet touches the gauge, some make it as large letter rate as long as they can push through the gauge.

If sellers charges letter rate and post office charges parcel rate it goes up to $6.60 for parcel under 500g. And the seller had to pay extra from own pocket. It has happened to me before.

If the book was charged as a parcel, with padded bag it would have cost about $8. Then $2 handling cost  was quite reasonable.

I do think the negative feedback is harsh and the buyer should have contacted the seller before leaving the feedback.