in checkout_process_payment find this line (or similar - I may have added the strip_tags call:
$params['gst_exempt_fields'] = $params['gst_exempt_fields'] .'Shipping Method: '.strip_tags($order->info['shipping_method']);
comment that line and see what happens. however, disabling that line will probably add GST to international shipping. best to have a test, if international destination, then run that line.
hope this helps