In QuickBooks Online (QBO), you can customize your invoice and other sales forms. However, adding a QR code at the bottom of the invoice is currently unavailable.
I can see how having this option would be helpful for you and your business. We'd love to hear your input. I recommend leaving feedback to notify our engineers about the feature you need. This way, they can consider adding it in future updates.
Here's how to submit feedback:
In QBO, go to the Gear icon at the top, then Feedback.
Enter your comments or product suggestions. Then select Next to submit feedback.