I would like to identify if a customer is a USA customer in shipping.php to make a decision on whether or not to add the Free Shipping weight back to $total_weight. Can someone direct me to the correct variable that would allow me to do this?

Something to the effect of:

if (USA customer)
$total_weight -= $_SESSION['cart']->free_shipping_weight();
else
$total_weight += $_SESSION['cart']->free_shipping_weight();

Thanks.