Hello,
In your Shopify Shipping and Delivery settings, what do you currently have set up? (screenshots help)
For something like what you are describing, a good setup would probably be to create 2 zones. 1 for USA and 1 for Canada. Then in each zone set up automatic shipping rates based weight and service type. You can also achieve this through conditionals grouping weights by common shipping prices. So for example like this:
This is an example of a setup using both conditional and service rates by weight with 2 zones (of course yours would look a different because I am located in the USA:
You may already have all of this set up the way you want it but let me know what you currently have set up so we can go from there and walk through the steps you need to take to achieve what you need.