You could customize the file:
/includes/modules/order_total/ot_cod_fee.php
and change the code at:
Code:
for ($i = 0; $i < count($cod_zones); $i++) {
if ($cod_zones[$i] == $order->delivery['country']['iso_code_2']) {
$cod_cost = $cod_zones[$i + 1];
$cod_country = true;
//print('match' . $i . ': ' . $cod_cost);
break;
} elseif ($cod_zones[$i] == '00') {
$cod_cost = $cod_zones[$i + 1];
$cod_country = true;
//print('match' . $i . ': ' . $cod_cost);
break;
} else {
//print('no match');
}
$i++;
}
and have the $cod_cost built based on a percentage rather than a flat amount ...