Leave it to me, @secgen42.

I'll help you get to the bottom of why that custom message isn't appearing in the Print Preview window. Begin by verifying the custom message was saved like so:

Changing Invoice Message to Customer

1. From QuickBooks Online, click the Gear icon (⚙) > select Custom Form Styles.
2. On the form you're using to send the Invoice > click Edit.
3. Navigate to the Emails tab > open the Standard email section.
4. Verify the message you created is still saved in the field that appears > click Done.
5. Navigate to the Invoice you'd like to send > Click Customize > make sure the template you verified the message on is selected.

Check out our extensive guide on setting up and changing customer addresses for more information. I'm attaching a video below which serves as a brief video demonstration of this:

Please don't hesitate to let me know should need any additional assistance, I'm always up to talk QuickBooks. Thanks for bringing us your question and take care.