when the checkout page posts to the confirm page, the shipping country is stored in a session variable named "eCartCheckoutForm_shipping_country".
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'