You could add to the triggers for the shipping charge:
AND
Based on session variables value:
if the session variable 'eCartCheckoutForm_shipping_country' has a value of 'US'
OR
Based on session variables value:
if the session variable 'eCartCheckoutForm_shipping_country' has a value of 'CA'
Ok, based on your instructions, I would guess that it would be better to remove the shipping price from the cart and add it instead to the confirm page based on the entered value on the checkout page.
I will give it a try. Sounds promising, thanks. :)