Can a user be set to use a default invoice template?
We currently don't support automatic assignment of custom form templates to specific customers. I recommend leaving feedback from within QuickBooks Online to ensure that the Product Team knows that there is a demand for this feature.
To give feedback:
Select the Gear icon at the top, then Feedback.
Enter your comments or product suggestions then select Next.
You'll be provided a list of suggested Help articles related to your comment. You can read through them or select Skip and send message.
From the drop-down, choose the appropriate category, then select Send message.