Thanks for following up on this thread, Knaght. I want to make sure you're able to use the customize option on invoices in your QuickBooks Online account.

Since the troubleshooting methods mentioned above didn't do the trick, I recommend reaching out to a member of the QuickBooks Online Support Team. Agents have specialized tools such as the ability to share your screen to better determine what's causing the issue and get it resolved as quickly as possible.

Here's how to get in touch with the team:

1. Click the Help button in the top right corner.
2. Select Contact Us.
3. Enter Support in the field and choose Let's talk.
4. From here you can select to receive a callback at a time that's convenient for you, or begin a live chat session with an agent.

The following article provides these steps if you ever need them again in the future: QuickBooks Online Support

Please don't hesitate to reach back out if you have any other questions or concerns. I'll be here to help in any way that I can.