Having the ability to see and apply any credits on an account when posting an invoice, that then also prints/emails with the total owing after that credit. If the customer has a credit, and you email/post an invoice, the customer doesn't see the amount owing minus the credit, and often pays the full amount still leaving that credit on the account.

Either that, or having a spot on end of invoice after the grand total that shows what is owing on account with the credits included.