Thanks for sharing the details with me, @rhfullerton1.
I can imagine how this issue affects your business. I have some steps we can perform to isolate this issue.
There are times that the browser is full of frequently-accessed pages. Thus, it causes unusual behavior to your QuickBooks Online (QBO) account. To verify, let’s access your account on a private window (incognito).
Using a private window doesn’t store any data. Thus, this helps us determine if this is a browser cache issue. To start, you can follow these keyboard shortcuts depending on the browser you’re using.
Once logging in via a private window, try to check with your customer if they still get email reminders from you. If they haven’t, then I’d recommend clearing your browser's cache. This will delete all data and make your browser to function efficiently.
If the issue persists, try using other supported browsers. That way, we can see and check if this is a browser-integrated issue.
If you're still having the issue after performing the steps above. I’d recommend contacting our Support team. That way, you’ll be added to the list of affected issuers and be notified via email of any news and updates about the issue.
Let me also attach this article that you can read for future reference: Customize invoices, estimates, and sales receipts in QuickBooks Online. This link can walk you through the steps on how to add information to your invoice.
In case you have other concerns or questions about invoices in QBO, you can always get back to me by leaving a reply on this thread. I’ll be around ready to help you. Keep safe