Kirim email kepada pelanggan saat membuat faktur secara terprogram

Saat membuat faktur secara terprogram, bagaimana saya bisa membuat sistem mengirimkan faktur kepada pelanggan?

$order=Mage::getModel('sales/order')->load($orderid); 
$invoice = Mage::getModel('sales/service_order', $order)->prepareInvoice();
if (!$invoice->getTotalQty()) {
    Mage::throwException(Mage::helper('core')->__('Cannot create an invoice without products.'));
}
$invoice->setRequestedCaptureCase(Mage_Sales_Model_Order_Invoice::CAPTURE_ONLINE);
$invoice->register();
$transactionSave = Mage::getModel('core/resource_transaction')
                   ->addObject($invoice)
                   ->addObject($invoice->getOrder());
$transactionSave->save();

Setelah $transactionSave->save();:

$invoice->getOrder()->setIsInProcess(true);
$history = $invoice->getOrder()->addStatusHistoryComment(
    'Programmatically created invoice', true
);
$invoice->sendEmail(true, '');
$order->save();

Ini akan menginstruksikan Magento untuk mengirim email faktur kepada pelanggan.