Hm, if most of your business is from the US, you may want to set United States as the default shipping country in the cart. You can do this in Seller Admin > Account Preferences.
Also, for PayPal checkouts where we can neither block completion of checkout nor recalculate shipping during checkout when the destination provided during checkout does not match that selected in the cart, we do email you a Notification of such a discrepancy, so before you ship the item, you can contact the buyer to see if they want to make up the difference in shipping charges or just cancel their order.
You may also wish to consider upgrading to PayPal Pro and/or adding Google Checkout to your payment options, so at least buyers who choose to pay that way would have their shipping recalculated accurately during checkout if the destination has changed there vs. their cart selection.