You can add a customer contact list in QuickBooks Online (QBO), Connie. Yet, selecting multiple emails from a dropdown isn't currently available. I can share some alternative methods that will help.
In the meantime, you can add multiple emails to the customer profile, invoice, and estimate page by separating them with commas. These listed addresses will receive an invoice when sent. Please see the sample screenshot below:

You can also consider using a third-party app to achieve the option to choose multiple email options from a dropdown menu. Here's how:
- Sign in to your QuickBooks Online account.
- Select Apps from the sidebar menu, then go to the Find Apps tab.
- Enter a keyword in the search bar or click the Browse category button to filter the results.

Here’s an article that provides detailed assistance on how to organize your customers: Manage customer list.
I'll encourage you to submit your valuable idea about adding a dropdown option to select multiple emails with our product engineers to consider this in future updates. Here's how:
- Go to the Gear icon in the upper right.
- Choose Feedback under Profile.
- Type in your recommendation in the Share your feedback field.
- Click Next to submit.
Additionally, you can run reports to get a comprehensive overview of your business' finances.
If you need assistance or have further questions about managing your customer details, please do not hesitate to reach us. We're here to help ensure your transaction is smooth and secure.