When an invoice is not paid before it's due date arrives, then the payment and the invoice is considered past due. The phrase feels awkward to use, is there another?
There is another word, "overdue", but I don't think that would fit well in terms of an invoice.
Best Answer
I think at this point the invoice might be considered "past due" though perhaps "unpaid" might be clearer but the payment is "overdue".
Perhaps the issue is that you are looking for one word to cover both ...
I'd suggest that "overdue" is a correct term for the payment and "unpaid" is a correct term for the invoice.