You should always make sure to backup your store before you make edits like this in case there are any problems. The idea is that you need to edit the cart and in the shipping tab add a new charge. You can base it on the same criteria as your US charge, but instead of checking the country to make sure it is set to US you need to check it to make sure that it does not equal US.
Other than this you just need to add in the $20 flat rate for the charge. Please post back and let us know if you experience any problems with this.