this would take some hand coding to accomplish.
the items Shipping price would need to be stored in the cart along with the other item details.
then you would need to add code to loop through each item in the cart and set a session with the highest shipping price.
For example, if the cart where named "ecart1" and the Shipping codest column where named "ShippingPrice", you could use the following code:
php:
<?php
$eCart1->GetContent();
@session_start();
$_SESSION['shippingCost'] = "0";
while (!$eCart1->EOF()) {
if($eCart1->DisplayInfo("ShippingPrice") > $_SESSION['shippingCost']) $_SESSION['shippingCost'] = $eCart1->DisplayInfo("ShippingPrice");
$eCart1->MoveNext();
}
$eCart1->MoveFirst();
?>
then you could create a shipping rule that uses the trigger If session variable 'shippingCost' exists.
and set the calculation to a flat rate of $_SESSION['shippingCost']
