I can definitely help you with that, allsurfacepainti.
You can create a custom field to add your ABN in your quotes or estimates. I'll show you how to do it:
Click the Gear icon.
Choose Account and settings.
Select the Sales tab, then click the Edit icon in the Sales form content section.
Under Custom fields, enter "ABN" in the Name field then tick the Internal and Public checkboxes to show the field in the customer forms and QuickBooks.
Click Save, then Done.
Alternatively, you can turn on the ABN option in your quote template. Here's how:
Click the Gear icon.
Choose Custom form styles.
Locate the template, then click Edit from the Action column.
Go to the Content tab, then select the header area of the form.
Under the Header section, tick the ABN checkbox and then enter your ABN number.