Sorry, but there is not a way in Auctiva to access Canadian shipping options if you have specified listing on the US site. You can't do that listing directly on eBay Canada either.
However, if you list on eBay Canada, per this link from eBay http://pages.ebay.ca/help/sell/intl-ov.html "Bids and prices on a listing automatically appear in both the currency you specify and the currency that's typically used for the country site on which the buyer is viewing the item. (Note that equivalence is approximate.)"
IF the item you are selling is going to ship from Canada, then you will need the Canadian methods. The link above gives full detail on options you have.
Hope this helps.