How to add products to recurring invoices

How do I track things to add to a recurring invoice automatically. In other words I have a recurring invoice for 1000 monthly and have to add different parts and hardware each month to the original 1000 monthly invoice? Right now I am just tracking it in my headand adding it manually. Is there a simpler way? I have looked all over the forums for an answer and I am sorry if this has been asked already.