I don't know about shipping within Canada in particular, so this might not be helpful to you, Studio52, but I made a notebook with all the shipping rates and info I could find for the USPS. I bought a digital postal scale on Amazon for under $30. Each time I'm writing a listing for a new type of item, I put it on the scale along with the box or envelope I'll be using and all the other items that will be in the package (thank you card, business card, bubble wrap, freebie item, etc.), and I use that weight to figure out shipping costs to US, Mexico, Canada, and Everywhere Else. For the Everywhere Else option, I just pick the most expensive shipping rate there is for the weight of my package and call it good.
I worried about the high cost of shipping for some of my heavier items, especially international shipping, but I just shipped out an order to Canada today where the item cost $20, and the shipping cost was $24. I guess when you want something, you really want it!