by support » Sun Jun 29, 2014 8:36 pm
No, there is no way because Combined invoices is just a read-only view of the individual source invoices. There is no "combined invoice" data per se, so no place for a fee to be stored outside one of the sources.
The only way to show the fee input fields on Combined invoices would be to have it just pick the first source to add the fee to. This would only really save you one step of clicking over to Invoices before adding the fee, and the result would be the same.