In ProPlanner, you can only delete the latest invoice of each fiscal year, unless it is a pro-forma invoice (not yet booked). 

Pro-forma invoices

You can delete any pro-forma invoice. 

Booked invoices

You cannot delete booked invoices (except for the last invoice of the fiscal year) because it would disrupt the automatic numbering of the invoices. And that would contradict the accounting rule that invoices should always be arranged chronologically. If you want to delete an invoice, but there are other invoices entered after that invoice, you must delete all invoices up to and including that date. The same reasoning applies to credit notes.