Hi saucykats,,probably because the seller hasn't a clue what you are talking about because he is totally unaware of the additional charge.
Seller is using ebay's global shipping program administered by Pitney Bowes...
http://pages.ebay.ca/help/buy/shipping-globally.html
Duty is not involved but PB is charging program fee cost to buyers. Totally wrong in my opinion for exempt items but it is what it is.
Sellers using GSP, when viewing listings in first section under shipping will see..import charges ..to be provided at checkout. Your choice to buy or pass.