Generate (create) automatically invoice after the orders are shipped

Create module config file : app\etc\modules\Nik_Autoinvoice.xml

<config>

    <modules>

        <Nik_Autoinvoice>

            <active>true</active>

            <codePool>local</codePool>

        </Nik_Autoinvoice>

    </modules>

</config>

I'd go the creating an event observer for sales_order_shipment_save_after:

File path: app/code/local/Nik/Autoinvoice/etc/config.xml 

<?xml version="1.0"?>

<config>

    <modules>

        <Nik_Autoinvoice>

            <version>0.1.0</version>

        </Nik_Autoinvoice>

    </modules>

    <global>

        <events>

            <sales_order_shipment_save_after>

                <observers>

                    <nik_autoinvoice>

                        <type>singleton</type>

                        <class>Nik_Autoinvoice_Model_Observer</class>

                        <method>autoInvoice</method>

                    </nik_autoinvoice>

                </observers>

            </sales_order_shipment_save_after>

        </events>

    </global>

</config>

Now, we create one observer file : 

app\code\local\Nik\Autoinvoice\Model\Observer.php 

<?php

class Nik_Autoinvoice_Model_Observer

{

 public function autoInvoice($observer) {

 $shipment = $observer->getEvent()->getShipment();

 $order = $shipment->getOrder();

 if($order->canInvoice())

 {

 $invoice = Mage::getModel('sales/service_order', $order)->prepareInvoice();

 $invoice->setRequestedCaptureCase(Mage_Sales_Model_Order_Invoice::CAPTURE_ONLINE);

 $invoice->register();

 $transactionSave = Mage::getModel('core/resource_transaction')

 ->addObject($invoice)

 ->addObject($invoice->getOrder());

 $transactionSave->save();

 }

 }

}