The application also supports post-paid users as well, the way you are managing now. You tell the software to create pre-paid cards, you sell the cards. Users can buy pre-paid cards and sign up for an account in the system without even talking to you right from any of the client machines you have!

After creating an account, a user can log into his/her account anytime on any machine as long as they have enough credit in the account. On credit expire, the user is automatically logged out of the system, then they can even recharge their account with another prepaid card!

There are post paid-users as well who also have accounts created by you and the system keeps track of the usage time for the post paid users too in order for you to bill them later. Client machines remain completely locked until a user logs in. The whole process is controlled from the server.