Thank you for taking the time to replicate this, Angela. Let's ensure we successfully add a permanent message to the Tell your customer how to pay you box on your invoices.
You can configure the option to add a fixed message to the Tell your customer how to pay you box in the Invoice payments section under the Sales tab. In this section, you can set your preferred payment methods and add a permanent message for your customers. Check out the image below for visual references.

Since this option isn't visible on your end, you'll want to ensure first you have the necessary user permissions. If user permissions are set correctly, proceed with the troubleshooting steps to determine if the browser is causing the issue.
Log into your QuickBooks Online (QBO) account using a private or incognito window. This will create a clean browsing session without stored cookies or history, which can help resolve problems caused by cached data.
Use the following keyboard shortcuts specific to your browser:
- Google Chrome: Ctrl + Shift + N
- Mozilla Firefox: Ctrl + Shift + P
- Microsoft Edge: Ctrl + Shift + N
- Safari: Command + Shift + N
If the private browsing session works, clear your browser's cache to refresh the system. Ensure to perform this action on a supported and updated browser for the optional QuickBooks performance.
Meanwhile, you can also enter the payment instructions in the Note to Customer box on your invoices. If you'd like to make a default message on this box, create a Custom form style and use this template when recording an invoice. I'll add an image of how you can configure this option.

Here are some helpful resources you can use to learn more about creating end statements to summarize customer accounts, setting up automated payment reminders, and entering prior balances for customers with outstanding payments:
The QuickBooks Community forum is always available to provide guidance and support, day or night. If you have additional questions about customizing invoices or managing your sales transaction, share them in the comments below.