Hi there,
I know how important it is for the email to auto-populate when you're copying an invoice. Don't worry, I'll be happy to share more info on this.
The default settings in QB Online don't allow for the email to auto-populate when copying an invoice. I encourage you to send feedback about this to our engineers by clicking the Gear icon and looking for Feedback. New product ideas are based to the valuable feedback we get from users like you.
In case you need further assistance, feel free to contact our support team using this link here.