Hi Michael!
Sorry, my bad with the code sample. Please replace this piece of code in your Core/Mailer.php file
Code:
public function sendProcessOrderCustomer(\XLite\Model\Order $order)
with the next one:
Code:
public static function sendProcessOrderCustomer(\XLite\Model\Order $order)
After that re-deploy the store and let me know if it works out.
Tony.