单页结帐后的Magento将新产品添加到购物车

i would like to add some Products after the Checkout I have an observer and it's listening on 'checkout_onepage_controller_success_action'

when i try

$this->_getCart()->addProduct($product, $params);

and save the cart the success page will be loaded but no new products in the cart.

Any idea what's wrong?

It's working if you know that you must do a new

$cart = Mage::getSingleton('checkout/cart');

before you can go one