Сначала добавьте код наблюдателя событий в ваш модуль config.xml /your_module/etc/config.xml,
<global>
<events>
<sales_order_invoice_save_after>
<observers>
<your_module>
<type>singleton</type>
<class>your_module/observer</class>
<method>sales_order_invoice_save_after</method>
</your_module>
</observers>
</sales_order_invoice_save_after>
</events>
</global>
затем добавьте наблюдателя событий в ваш Observer.php
/your_module/model/Observer.php
Class NameSpace_Module_Model_Observer()
{
public function sales_order_invoice_save_after($observer)
// here should be save_after because invoice id not available until the object has been saved that means an order
{
$invoice = $observer->getEvent()->getInvoice();
// to see what variables available in $invoice, use the following
// echo "<pre>"; print_r($invoice->getData()); exit;
}
}