How do you know the buyer did not ask about the grossly inflated postage charge before leaving the FB?

Because OP stated There was no email to discuss their concerns, just straight to the negative feedback!

The book is 19.1cm x 12.7cm. Considering the thickness of the book I would say Padded Bag Pb3 $1.30 was most suitable to be used.

As the book was very old and probably correctable I would wrap the book before putting in the padded bag if I were the seller. It could cost a few cents. I think the buyers estimate $2 for packaging is pretty accurate.