Hi there, @bbmhayes.

Thank you for the update and for providing additional information. 

We understand that you'd like to disable the option for customers to reply to invoices and estimates sent through QuickBooks Online. However, there is no specific option available to turn off this ability. 

To work around this situation, you'll want to update your customer-facing email address to the desired email. By doing this, you can ensure that customer responses are directed to a designated email address that you've set up for communication purposes. This will help you manage customer inquiries effectively and ensure that the right person receives and responds to their messages.

Here's how:

1. Navigate to the Gear icon in the top right corner and select Account and Settings from the drop-down menu.
2. Choose the Company tab on the left-hand side.
3. Look for the Contact info section and click the Edit icon.
4. Next to Customer-facing email, uncheck the Same as company email box.
   
5. Change it to the desired email address.
6. Select Save, then Done to save your changes.

I also encourage you to add your vote for this one by sending a feature request in the Feedback section within QuickBooks. Our product development team reviews all the feedback we receive to ensure we’re meeting the needs of our customers.

Here's how:

1. Select the Gear icon at the top, then Feedback.
2. Enter your comments or product suggestions then select Next.
3. You'll be provided a list of suggested Help articles related to your comment. You can read through them or select Skip and send message.
4. From the drop-down, choose the appropriate category, then select Send message.

Additionally, you can change the email address that appears on your sales forms. 

Feel free to post here in the Community if you have any concerns about sending invoices. We're always here ready to help. Have a good one!