Welcome to the Community, @mfrancis1. I appreciate you raising your query here in the forum.

I recognize how convenient it is to have the past balance due shown in the invoice in QuickBooks Online (QBO). Let me guide you through the steps on how:
 

1. In your QBO account, go to the Gear icon, then select Custom Form Styles.
2. Locate the custom invoice that you use, then click on Edit.
   
   
    
3. Go to the Content tab, then click on the Table.
   
   
    
4. Tick the Show on invoice box.
   
   
    
5. Once ready, click on Done.

If you wish to customise your sales forms more, you can refer to this article for detailed information: Customise invoices, estimates, and sales receipts in QuickBooks Online.

Moreover, I'll also share this article that can serve as your reference if you want to give a discount to your favorite customer in QuickBooks: Add a discount to an invoice or sales receipt in QuickBooks Online.

Feel free to leave a reply if you have additional QuickBooks-related queries. The Community team always has your back. Have a good one.