QBO - How to show on current invoice the previous unpaid invoices or outstanding balances?

You can add the account summary section, Ddu14. It will help you show the outstanding balances.

You'll have to edit the invoice template to achieve this. I'll help you out. 

Here's how:

1. Click the Gear icon.
2. Go to Custom form style.
3. Select the invoice template, then click Edit.
4. Go to the Content tab.
5. Under Account summary, tick the Show on invoice box.
6. Click Done.

Although, the summary will only show when you print preview or email the invoice.

You can also refer to this article on how to customise invoices, estimates, and sales receipts in QuickBooks Online for more information.

If there's anything else that you need, feel free to reach out to us again. Keep safe!