To MBilling send a invoice, add this command in the linux CRONJOBS.
php /var/www/html/mbilling/cron.php invoices
In this case MBilling will use the user field 'Creation Date' to calculate the INVOICE.
MBilling will send the invoice in PDF to user email.
To change the logo in the invoice. Change the image logo.png in directory /var/www/html/mbilling/protected/views/invoices/
Remember: To send email, you need configure your SMTP in Configuration Menu.