Friendly first reminder
A day or two after the due date
Subject Quick reminder: invoice [INV-0000]
Hi [First name], A quick note that invoice [INV-0000] for [$0,000] was due on [due date]. If it has already gone out, thank you, and please ignore this. You can see the invoice and how to pay here: [invoice link] Thanks so much, [Your name] [Your business]