You'll be glad to know that QuickBooks Online can help you gently remind your customers about their overdue invoices by setting up a reminder email in your Accounts and Settings page. Let me share the steps on how you can do it.
Go to the Gear ⚙icon.
Select Accounts and Settings.
On the Sales tab options, select Reminders section, click the pencil to edit.
Select a greeting from the dropdowns.
Edit the subject and email message. You can also stick with the default message. Tip: We recommend you keep "reminder" in the subject.
When you're done, select Save and then Done.
This applies to all customers and QuickBooks also uses the template in your setup for all invoice reminders. When you create an invoice, QuickBooks check your invoice due dates every night at midnight. Make sure you always include en email address in the Customer Email field on invoices.