Invoicing

I have a VoIP system where the users have a account and can use this service. After refilling the user balance (paypal, moneybookers or per hand from admin) no reciept (invoice) is genereted from the system. This system must be completed with a script that generates promptly an receipt (pdf) after the refilling and safe it in a DB. I need a user and an admin interface for this invoices.