Hmm... Without getting too much details on what you are offering, I would presume you are selling a service rather than a product. Here is something you can do, instead of having the recurring billing tasks performed on the shopping cart side. I would recommend using Authorize.Net ARB (Automatic Recurring Billing). It's a little more labor intensive because you need to manually setup the each subscribtion, price, billing interval, and etc. However, it will save you alot of headaches than trying to do this from your server.
Example, typically recurring billing involves the following, need to generate invoice first, then for those clients/account with a balance you need to submit transactions to a payment gateway. But typically there are many variables involved when doing recurring billing, in the event when a credit card declines you need to have ways to specify when to re-attempt these transactions or you or the client will be notified. This is just one example.
Althought Authorize.Net is not perfect but I think it should be able to handle most of your recurring billing needs.
what i need is s shopping, when a client sign up, it can send invoice automatically to the client. Since the product/service are to be paid on monthly installment, it must be able to send email/invoice to the client every month. And the admin side can click on every invoice to update it as paid.