It would be helpful to have an email history of when the automated past due emails are sent as well as the MLS Administrator being cc'd on those emails. Another place they could be recorded is at the bottom of the invoice under the Invoice History.
I would like to be able to take overpayments and apply them to the group's overall balance. From there balance, I would like to apply payments to their future invoices.
I love that you've added an Estimate tab. Would it be helpful to show on that tab when that estimate was sent, emailed, and the email address it was sent to? Perhaps at the very bottom of the estimate in view mode?