In our Sales screen, we have one customer whose invoices always say that there's a Delivery issue, but we have contacted the customer and they are receiving the Invoices

Thanks for promptly informing us of the issue you're experiencing with QuickBooks Online "Undelivered" to invoices sent and received by your customers, @dnmh. I'm here to help.

If you have selected (Blind copy) Bcc in the Accounts and settings when sending invoices, we are currently investigating this issue (INV-96334) where invoices show undelivered but invoices are received by customers. In this case, I recommend contacting our Support team to add you to the list of affected users and be notified promptly once there's an available fix or if the issue has been resolved. 

If you have not selected the Bcc option, this may be a temporary cache-related problem, and I recommend performing browser troubleshooting steps. Let's first access your account through your browser's Incognito mode and see if QuickBooks Online still prompts you undelivered invoices notification when going to your Sales screen. 

• Google Chrome: Ctrl + Shift + N
• Mozilla Firefox: Ctrl + Shift + P
• Microsoft Edge: Ctrl + Shift + N
• Safari: Command + Shift + N.

When the delivery notification stops when going to the Sales screen, you can proceed to clear your browser's cache. This process will remove and fix specific issues and help the program run optimally. Additionally, you can use other devices and supported browsers to ensure everything works as expected.

If you're using QuickBooks Payments, let me add this article in case you encounter issues applying payments to customers who pay online: What to do if your customers can't pay their invoices online.

We're always here in the Community should you have further updates on the issue you experience with undelivered invoices despite your customer receiving one. We'll do our best to assist. Take care.