Freeradius Billing Account Management

Billing Account Management ======================= I 've successful about installation freeradius + mysql + dd-wrt ( using chillispot ). Now, i need you help. Would you like to suggestion me about "Hotspot Billing Account Management" ? that can provide postpaid/prepaid and register/sign up new user and can be refill internet time . Anybody have experience about this ? Thank you very much Best Regards., PT Ps. have it a opensource ?