I don't think it's an "international fee" - it's most likely VAT or sales tax. I just checked my last Kobo purchase, and sure enough they seem to have added French VAT of 19.6% to the price of the book. (Still cheaper than the Sony store in the US, though.) They've never asked me for an address, so I suppose they're using the IP address you log in from to determine what rate to charge you.
I've long suspected that the Sony store simply doesn't want to deal with foreign VAT rates and that's why they rely on either the billing address of the credit card you use or the physical address you give them when paying by gift card.