Thanks for posting to the Community, @ckchew.

Currently, adding a QR code to invoices isn't supported in QuickBooks Online.

I recommend checking the Intuit Marketplace to spot one. The gateway is a one-stop-shop where you'll find apps that combine with QuickBooks.

If you want to learn how to send invoices over time as you work off an estimate in QuickBooks, you can review this article: Set up and send progress invoices in QuickBooks Online.

Please keep me posted by leaving a message to this post if you have questions. I'll be around to help. Take care!