Why are my customers receiving reminder for invoices already shown as paid?

Hello Userpete3, thanks for posting on the Community page, we have had no other reports about this come to us.  The reminders are only sent when invoices are still open/overdue,

so it is odd that it is sent even if the invoice is already paid. I can only think of a user manually sending invoice reminders. Have you checked to see what the audit log says for the invoice reminders sent.  If still having an issue we would advise you to reach out to support either via phone at 0808 234 5337 or via chat so that you can set up a screen share so it can be looked into more for you.