You can get a Total of Entries, Expense and Duration on the bottom of the invoice. Setup -> Invoicing -> Export Settings -> Total: Entries, Expenses and/or Duration. (in TM v4.1 it will be Setup -> Invoicing -> Companies -> (my company [per company]) -> Export Settings -> Total: Entries, Expenses and/or Duration).
It will not line up under any columns though. First it would be confusing, second since there could be three things, we can't make them all line up under the same column as that would take too much vertical space. Last we need to add the labels so you know what they are for. Hope that makes sense.