I'd be glad to help you with this, @Kathleen03.

Regarding the missing "Message on invoice" section in the invoice you've sent to your customer, your customer may need to use an incognito browser to see the message. It is located at the bottom part of the invoice. I've also replicated both your concerns about the invoice and contacting our support team and I find no problem with it.

I've added a screenshot for additional reference.

I'd suggest performing the browser troubleshooting steps to see if this is a data-related issue. You can share this method with your customer. Start by using an incognito browser to avoid using data. Then, let them open their email and check on the invoice if the message is there. If yes, they can clear the cache. An alternative way is to use another browser.

The method above also applies when you're trying to contact support. This is done because over time the data in your browser will become outdated and may cause any odd behavior. Here are the keyboard shortcuts to access an incognito browser:

• Google Chrome: Ctrl + Shift + N
• Microsoft Edge & Mozilla Firefox: Ctrl + Shift + P
• Safari: Hold down ⌘ + Option + P 

I've included this article for your future task whenever you'd like to personalize your sales forms: 

Customize invoices, estimates, and sales receipts in QuickBooks Online.

You're always welcome to get back in the comment section below if you need further assistance or questions. I'd be glad to help you through it. Take care.