I've made a custom shipping calculation, but now I need to remove the Shipping Amount on the pdf invoice. I'm using the Symmetrics Invoice Pdf, and I've tried to copy

app/code/core/Mage/Sales/Model/Order/Pdf/Abstract.php to app/code/local/Mage/Sales/Model/Order/Pdf/Abstract.php

and changed

 $totalShippingChargesText = "(" . Mage::helper('sales')->__('Total Shipping Charges') . " "
            . $order->formatPriceTxt($order->getShippingAmount()) . ")";

to

 $totalShippingChargesText = " ";

but it still shows the shipping costs on the invoice pdf. I don't have a getShippingAmount in my invoice.php (app/code/local/Symmetrics/Invoicepdf/Model/Pdf/Invoice.php).

The module can be find here: https://github.com/symmetrics/mrg_invoicepdf

So where do I have to look?