I would like to see this feature added as well. It would be a lot easier, a lot of clients do not appreciate it when their invoice is reissued with the same invoice number however with modifications. It also makes it hard for accounting/tax reasons (especially in Australia, I'm not sure about other countries) as once an invoice is issued we are not supposed to increase the amount on the invoice and unfortunately by applying late fees in the way they are currently being applied would easily lose in court if it ever got that far.

I know my example is a bit far fetched, however it is still true.

The ability to add late fees to future invoices would be convinient and is the method used by most companies and a few other billing systems.