You can generate invoice and send it as attachment in email. All you need to do is call the sendEmail function. You can define the subject, message, from, cc, bcc information in sendEmail function.
$xinvoice = new Xinvoice();
/**
* Set email related settings for xInvoice
* @param array $to To email
* @param string $subject Subject of email
* @param string $message Message of the email
* @param array $from From email
* @param string $cc CC email
* @param string $bcc BCC email
* return object Object of class
*/
$xinvoice->sendEmail(array("to_email@gmail.com"),"Invoice Subject", "Attached Invoice message",
array("from_email@tech.com"));
$xinvoice->setSettings("output", "F");//output type must be F to send as attachment
echo $xinvoice->render();