Back End Billing Program

I'm looking to have a back end billing program designed which will have a web based front end and Microsoft SQL Server database back end. Let me explain more about the business so you can get a better idea of what I'm looking for and to accomplish. My company is an online backup solution which resells a backup solution product. Here's how I see the flow chart. Customer goes to our website, they click on the sign up page. When they fill out the sign up form and click the submit button, the data gets written to the SQL database. An auto generated email will be sent to the customer thanks them for signing up and giving them all the instructions on getting started. Also we'll receive an email notify us a new customer signed up and send us their information so we can call them. I would like the backend billing program to be all web based. Within the SQL database I will specify all the field needed for each item. At the end of each month, when I receive the report of the total gigabytes each customer used, I would log into the web based program and enter in the values of totat gigabytes used for each customer. When I'm done, I want to run a report that will automatically generate an invoice in PDF format to each customer and email it out to them with the total they owe. If possible I would like to have a paypal button on the email which they can buy the total amount. As each customer pays, we will log into the program and credit their payment for that month. From our website I would also like the customer the ability log into their account and see their payment history and data usage. I'm going to find out from the backup vendor if there's a way to import the monthly backup data report into the SQL database so it's an automated process.