Thanks for getting back, to @CB247. Let me assist you with sending an invoice to multiple email addresses of your customers.

In QuickBooks Online, you can easily add additional email addresses to a customer's profile. This way, you can set a default email address to use when sending an invoice to that customer.

Here’s how to add an email to your customer:

1. Go to the Sales section, then select Customers.
2. Choose the customer for whom you want to add an email address. 
3. Click on the Customer Details tab.
4. Select the Edit option.
5. Enter the email addresses separated by a comma and space.
6. Once done, click on Save to save the changes. 

You can also read this article for the complete details: Email a sales form or report to multiple email addresses.

Furthermore, check out these some helpful links on how to accept electronic payments from customers for online invoices or in-person sales using QuickBooks: 

• Record invoice payments in QuickBooks Online
• Take and process payments in QBO.

Feel free to return to this post for any further assistance with sending your invoices in QBO. I am always here to help.