<?php
Am_Di::getInstance()->hook->add(Am_Event::THANKS_PAGE, 'onThanksPage');
function onThanksPage(Am_Event $event){
    /* @var $di Am_Di */
    $di = Am_Di::getInstance();
    /* @var $invoice Invoice */
    $invoice = $event->getInvoice();
    /* @var $controller ThanksController */
    $controller = $event->getController();

    $controller->redirectLocation('/url/where/user/should/be/redirected');
}