Before the due date, keep the message helpful. After the due date, name the outstanding balance. In later follow-ups, include the date of the previous reminder and the next practical step.
Subject: Invoice #1042 — $2,370, due 12 September
Hi Dana,
Invoice #1042 for $2,370 was due on 12 September, and I have not seen the payment come through yet. It may be sitting with your accounts team, so I have attached it again.
Could you confirm that it is approved for payment and let me know the date it will go out? If anything is needed from my side, I can send it today.
Thanks,
Alex
The subject identifies the invoice, the first sentence gives the amount and due date, the message leaves room for an approval delay, and the final question asks for one concrete next step. Replace every example detail before sending.