How to group Projects Billing Events into separate Individual invoices

On our contract projects we use Events for Invoice Generation, Our events are created through an interface from External System. Our requirement is to group a set of events into one invoice when invoice is generated.

For Ex: In Our business scenario: We have planned shipments for which Billable Events are created on a project. When events are created for 2 different planned shipments(captured on an attribute or reference on the event) at the same time. Our requirement is to have 2 different invoices(one per planned shipment set of events) created.

Is there a way to control/group events so that they are grouped into an invoice as above? Any billing extensions or Set of Invoice Formats?