found the answer here: http://www.zen-cart.com/forum/showthread.php?t=36421&highlight=international+handling+fee
(since i don't know the exact way to call the variables i decided to do a quick search instead of give myself a migraine :))
So.. if you want it strictly for USA.. here's the code for the USPS module (view that thread to see where to change it)
/// ADDED Extra Handling Fee for International Orders
if ($order->delivery['country']['id'] == SHIPPING_ORIGIN_COUNTRY) { // equal to the US?
$modified_usps_handling = MODULE_SHIPPING_USPS_HANDLING;
} else {
$modified_usps_handling = (MODULE_SHIPPING_USPS_HANDLING + 5.00); // my added handling fee is $5
}
///
$methods[] = array('id' => $type,
'title' => $title,
'cost' => ($cost + $modified_usps_handling) * $shipping_num_boxes);