Good day, galteecivil.
It's nice to see you here! Let me show you how to add the customer's ABN to the invoice. Please follow these steps:
- Go to the Sales or Invoicing tab and then click Customers.
- Select the customer’s name and click Edit.
- Enter the ABN in the Tax reg. no. field under the Tax info tab.
- Click Save.
Once done, let's customise the invoice to appear there.
- From the Gear icon, select Custom Form Styles.
- Find the invoice template you use and click Edit.
- Click the Content tab, then select the top box of the preview page.
- Scroll down and tick the box beside Customer ABN under the Display section.
- Click Save.
I've added this link for your reference: Customise invoices, quotes, and sales receipts in QuickBooks Online.
Post again in the QuickBooks Community if you need more help. Stay safe!