Really, I don't think it is Kobo's responsibility to tell you how your bank may have decided to make extra bucks off international sales. I am betting it is in the fine print that goes along with the offending credit cards.
US taxes show up too, even though some US companies don't charge taxes for on-line purchases. I don't know if that is a result of US regulations for International sales, or Canada's.
I'm an American with dual citizenship who has only lived in Canada for the past ten years. I remember being somewhat surprised to find out that other countries don't design their systems around what is necessarily most convenient or the least expensive for people in other locations. It happened when I ordered things from US companies, whilst living here
