Hi seseroy,
You may want to have a look at Spindle Professional from Draycir, www.drayicr.com/spindle You will be ble to send all you invoices in one go for multiple customers. Each customer will recieve their own email's with multi attachments at the click of a button, at the same time Spindle will automatically create a PDF archive library on you pc or network.
Contact me for more info,
Regards
Richard Haywood