Nudge 1: The friendly reminder
Send this two days before the due date. Keep it short: invoice number, total, due date, and payment link.
This note simply helps the client plan. Most will pay on time after a clear reminder.
Nudge 2: Due today
On the due date, restate the amount and make the payment link the most visible element in the email.
Include a line that says, "If you already took care of this, thank you." It preserves goodwill.
Nudge 3: Past due, still polite
Two business days after the due date, send a gentle overdue note with the original invoice attached.
Ask if anything is blocking payment. You are giving them a path to respond without embarrassment.