eCart creates a session variable for the shipping country called "eCartCheckoutForm_shipping_country"
You need to create 2 shipping rules:
1) US Shipping
triggers
If Session Variable Exists "eCartCheckoutForm_shipping_country"
AND
based on session variable value
If the session variabel "eCartCheckoutForm_shipping_country" is "=" a value of "US"
Calculation:
Flat rate of 18
2) Outside US
triggers
If Session Variable Exists "eCartCheckoutForm_shipping_country"
AND
based on session variable value
If the session variabel "eCartCheckoutForm_shipping_country" is "!=" a value of "US"
Calculation:
Flat rate of 25