Billing and Invoice CMS
Recently I acquired a new client that has me doing web development for them -- pretty basic stuff, integrating an existing web design with WordPress. However, my client recently approached me about setting up a system whereby they would have the ability to allow customers to securely create an username and password with an existing account number, log-in and view/pay their bills.
Therefore, I am wondering if anyone knows of any good CMS that could be employed to meet the above requirements. Also, keep in mind that my client is a small-town, medium-sized energy provider and needs a scalable solution (currently they have about 20,000 customers) They also have a relatively complex billing situation and would be willing to pay for another company to maintain a platform making the above possible.
Any suggestions are welcome!