flat_rate_filter.php

<?php
//Add One handling fee for any quantity
add_filter('shopp_cart_fees','flat_rate_filter');
	
function flat_rate_filter ( $fee ) {
	$shipping = shopp('cart.get-shipping','number=on'); 
	
	$products = array();
	while( shopp('cart.items') )
		$products[] = shopp('cartitem.get-product');
	
	if ( in_array(1062, $products) && in_array(2126, $products) )
		$fee = $shipping + 24.69;
	
	return $fee;
}