Multi-Country Price Differences in Shopping Cart

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J have businesses set up in Canada and the US but use the same website. Being most of the products come from the USA, I can list them at US prices, however if a customer is from Canada, in a lot of cases there are additional duties and freight charges applied when I bring products into Canada that I have to figure into the prices. Currently I have this set up through the currency view in oscommerce to show the difference but when it comes to checkout it uses the default US price. Is there a way to set the shopping cart up to charge prices based on the prices listed for country. OK, here is an example of what I am trying to explain

An item that I retail for 15.00 and costs me $12.00USD in USA is brought up to my Canadian business, to this I have to add additional freight costs, duties if product was manufactured outside the USA and brokerage frees. Now using the going bank exchange rate, my cost for the same $12.00 product up in Canada is now $14.05USD so if I want to make the 25% mark-up, my Canadian Price has to be the equivulent to $17.56 USD

Now when Canadian buyers view my website using the Canadian currency button, they see the Canadian equivelancy to $17.56USD but when they make the purchase they are only charged the $15.00USD price. Is there any way to correct this or do I need to set up a seperate store for my Canadian customers. Any suggestions or help with this would be greatly appreciated