What i suggest as a new feature would be very easy for you to implement. Alot of the people i invoice each have specific payment terms which they dictate.
These can be 7 days, 14 days, 30 days, 45 days, 60 days, due on receipt, or custom.
It would be very handy to have as part of the client information what that specific companies payment terms are. So when an invoice is created under the invoice date box it shows the payment terms for the company being invoiced and then the due date can be automatically calculated for the correct 30 days or whatever.
What do you think?