Handling Fee as Percentage?

[jmcdog]

Zen Cart 1.3.7
My client wanted to have a 20% handling fee. They are using UPS.
Here's what I did:

includes/modules/shipping/ups.php

changed this line:

'cost' => ($cost + MODULE_SHIPPING_UPS_HANDLING) * $shipping_num_boxes);

to:

'cost' => ($cost + ($order->info['subtotal'] * MODULE_SHIPPING_UPS_HANDLING)) * $shipping_num_boxes);

Then in admin: Modules >> Shipping >> UPS I set the handling to .2