Currently WHMCS has the ability to generate an invoice x days before the "Next due date". This is great.
The only issue is that the invoice is only generated and isn't charged. To make the system more flexible for invocing. I suggest there also be another field that says charge invoice x days before due date.
So for example. I have a monthly webhosting service that renews on on 1/7/08.
I have the invoice set to generate 7 days prior to the renewal which should be 1/1/08. I would also like the invoice to be charged "7" days prior to the "next due date". This would allow for alot of hosts to manage payments beforehand so if the service by the time the due date has arrived- hasn't been paid. It can be set to cancel "x" days after the due date. Most hosts are going to this model whereby they invocie AND charge "x" days prior to renewal and if by the time the renewal date hits, the service hasn't been paid, it will suspend the account. If creating a field that can have an editable date, how about something like a checkbox that says "charge on invoice generation".
The first options would of course give greater flexability.