Hi,@roxmmartin-gmail. Yes, it is possible to add multiple email addresses for a single customer when creating estimates or sending invoices. Simply separate each email address with a comma, then add another email address.
Here's how you can add multiple email addresses:
- Navigate to Sales and choose Customers.

- Select the customer you wish to modify. Then click edit.

- In the email section, enter the customer's email address followed by a comma.

- Then Save.
Please note, that when sending email transactions to your customers, all the email addresses you entered in their profile will show in the transaction.
If you want, you can manually remove all the email addresses you entered in the email transactions.
For additional information on how to edit your customer's details, please refer to this article: Edit customer info in QuickBooks Online.
For guidance on customizing sales forms, please refer to this article: Customize invoices, estimates, and sales receipts in QuickBooks Online.
We want to ensure that all your concerns are covered, if you have any additional questions, simply reply to the thread below and we'll be here to assist you. Best wishes!