You need to install and configure the billing system for VPN

Hello, have a dedicated server, it is used as a VPN, you need to install a simple billing system for users.
Basic requirements for billing:
1. A personal account of the user with the ticket, displaying balance, the ability to change the tariff.
2. Automatically disconnect the user when the balance is zero, with the appropriate notification in your personal area.
3. Support for PPTP and L2TP/IPsec (MPD5).
As simple system with basic functionality, preferably free but will consider all options.