I'm trying to create an intercompany billback with a billable expense but I can't get the balance due to display on the "invoice"

You've come to the right place, @jack-laird-bigst.

I can help you show the total balance due on an invoice in QuickBooks Online (QBO). The steps to do this are very simple. All you need to do is to edit the invoice template that you're using.

Here's how:

1. In QBO, go to the Gear icon at the top right
2. Select Custom Form Styles.
3. Locate the invoice template you use, then tick Edit ✎. 
4. From the Content tab, tap the body section or middle part of the preview.
5. In the Account summary section, place a checkmark on the Show on invoice box.
6. Hit Done to save it.

Once done, you'll see your customer's total due when creating and sending a new invoice. You may also refer to this article for further guidance: Add an account summary to an invoice in QuickBooks Online.

For additional reference, here's how to customize sales forms and reports in QBO to make them more presentable.

Feel free to get in touch if you have other concerns. We're here to help you out.