Welcome to the Community, @colinede9-gmail-.
I understand how important it is to keep track of your customers invoices. I’ll share some information about your concern about sending invoices in QuickBooks Online (QBO).
Currently, there’s an open investigation about the invoice that shows an undelivered when sending an invoice to the customer. Thus, our engineers are still working on a permanent fix for this issue.
For now, you can follow the troubleshooting steps in this article to get this fix: 3 solutions for you when customers aren't receiving your emails.
If the issue persists, I'd recommend contacting our QuickBooks Online Customer Care Team. They have the tools to verify your account and add you to the list of affected users. Then, once resolved, they will send you an email about this issue.
Please ensure to review their support hours to know when agents are available. This way, you can contact them at a time that is convenient for you.
When your customers receive the invoices successfully and processed their payments, you can record them following the steps from this article: Record invoice payments in QuickBooks Online.
Please let me know how else I can help you with your invoices by adding comments below. I've got your back and am always happy to help. Have a good one!
