How can we improve WHMCS?

We bill customers NET30 so that they have 30 days with which to mail in a payment before the due date.
We would like to see a feature where we can have a different number of days generation of an invoice in advance, based on type of payment.
For example, whereas Check Payments would be NET30, we would like to charge credit cards 7 days in advance of the due date.
Currently if we choose 30 days in advance it affects customers paying both by check and credit card.