Can QuickBooks send alerts to remind us when an invoice is outstanding?

Hello, dave89.

The ability to send you alerts in QuickBooks Online is unavailable. You can view the paid,unpaid and overdue invoices within QBO via the Dashboard.

Here are screenshots that serve as your visual guide:

You may also consider looking for a third-party app that do this task for you. You can go to this link: https://apps.intuit.com/app/apps/home.

Please tag me directly if you need more help with the process. I want to ensure this is taken care of.