I also have this problem and I think that the best way to solve it is to be able to generate the billable item with default "Invoice Action" -> "Add to User's Next Invoice"
So for example,
On a hosting account that will be invoiced "lets say" in 4 months from now, and overused 10MB of disk space last month I am 90% sure that next month he will be overusing 15MB and so on...
My control panel send him an email notice about overusing disk space... But I really don't like to send to the costumers invoices for 0,15€...
So why not adding all the generated billable items on his next invoice?