I recently received a complaint from a customer re: invoice. QB System administration is re-sending invoices multiple times, even before due date. How do I stop it?

Thanks for reaching out to the Community, @michael139.

Turning off overdue invoices notification in QuickBooks Online (QBO) is pretty quick and easy. This can be done by going to the Account and Settings.

Here's how:

1. In QBO, go to the Gear icon at the top right to get to the Account and Settings. 
2. Select the Sales menu at the left pane, then tick the Edit (pencil) icon in the Reminders section. 
3. Toggle the Automatic invoice reminders to turn it off.
4. Hit Save and Done to complete.

To learn more about how this feature works, consider checking out this article: Automate invoice reminders in QuickBooks Online.

I also add this article for more information about managing invoices in QBO: Learn how to create and send invoices.

Please know that I'm only a post away if you have any other issues or concerns. I'll be more than willing to help. Have a good one!