set up 2 shipping charges using the Based on session value trigger:
If session "eCartCheckoutForm_shipping_country" is "=" a value of "US"
for the flat rate and:
If session "eCartCheckoutForm_shipping_country" is "!=" a value of "US"
for international, this one will use the charge:
Based on cart Subtotal:
Cart Subtotal "Times" "0.4"