$zco_notifier and Paypal

[fotofx]

I am trying to create a process to run after checkout success. The following code works fine for all payment methods EXCEPT Paypal IPN.

class Dropship extends base {

    function Dropship() {

        global $zco_notifier;

        $zco_notifier->attach($this, array('NOTIFY_CHECKOUT_PROCESS_AFTER_SEND_ORDER_EMAIL'));

    } 

Any thoughts on how to get it working with Paypal IPN?

Thanks