Combining Invoices

My business generates monthly bills which have a fixed set of services (sort of like voice mail for your cell phone) and a variable set of services (like the call minutes on the cell phone) on them.   We don't currently subscribe to advanced billing because even the fixed set of services varies relatively often, so there are no set-it-and-forget-it billing schedules.  I'm working to automatically create an invoice with the variable services on it, and I plan on using a memorized invoice for the fixed set of services.  However, what I'd like to do is combine the two invoices so