Hi there, merryfield. 

I can see how beneficial to add variable on invoices in QuickBooks Online (QBO). As my colleague mentioned, the feature you're trying to access is unavailable.

To help you get through this, I'd recommend sending feedback to our Product Development Team. This way, once they see your recommendation, they'll consider adding it to the next program update.

You can follow the steps provided by my colleague above on how to submit feedback. 

In addition, utilize this article that can serve as your guide on how to customize invoices, estimates, and sales receipts in QuickBooks Online

Keep your post coming if you need more help about managing your invoices in QuickBooks Online (QBO). I'm always here to help.