Actually, I do not know if I have used the right terms to describe our situation.

Basically, We would like to have a system to help us to do billing and support at the same time. For example, we would like to offer outside users a customer support with ticket system, however the outside users need to purchase some credits beforehand through the system in order to submit their requests. Sometimes, our representatives will charge extra unit of credit for solving some open tickets due to the complexity and hours spent. (It will be nice to submit user a further billing request to same open ticket, and user can either accept/declined)

Is there any available application or service we can use for the purpose above?