Shipping charges calculations on a case by case basis
I am using PayPal as payment method for my customers on my webpage. Shipping charges for national customers are not a problem and are currently included in the price charged by PayPal to the customer. In case of international shipping addresses, the shipping charges should be calculated on a case by case basis. How can i add an extra charge to my international customers once they have already sent their order? or how can i make sure that only national customers can go ahead with the PayPal payment?