Occasionally we have problematic clients that we need to follow up with several times and remind them to pay their invoice. It would be great to be able to add simple updates at the bottom of an invoices (that only we can see). Something like:

DEC 7 18 , 5:12pm - Called Carol to remind her about invoice and she confirmed it was transferred to the billing department and they would handle it on Wednesday

DEC 14, 2:30pm - Got a callback from Carol apologizing about the late payment and she sent a messenger with gold bars in the amount of the invoice.