Department allocation for customer product invoice

SUGGESTED

I cannot find a way to include a department number on sales invoicing screen, there is only a field for the carriage. So when I want to run a department P&L, it's so frustrating that the sales are not appearing (only carriage!).  

Is there a way to resolve this, considering that the customers have a default department number allocated and therefore as per batch invoice screen, when I select a particular customer, the default department should appear too?