Accounting addition

Hi just need to attach an accounting system which will generate a monthly e mail invoice to registered people on our site, system needs to email a monthly invoice with an admin ability to override amounts, each month if admin does not check off payment within the system a penalty fee is added and resent every seven days until admin checks off payment, each invoice will have the current user ID as reference and a list of the email transactions eg date and amount and invoice number per user will be seen on their details when logged in. The site is [login to view URL]