How to achieve the Task wise billing

Hi All ,

As per the project setup ,  the client has created Project WBS based on the Item , so all the  items are created as task in the projects . so now they wanted to do the billing for the individual items based on events .

now the issue is in case user wanted a separate invoices for 10- 15 tasks when generate at a time .. i have checked by holding the event but that is very time taking if user needs to go for billing during the month end when the invoicing done in all the projects .

Can any one guide me how to do the same .