Hi, @bmac462. 

I know how essential it is to be able to determine who viewed the invoice, as well as enable the feature for the main recipient only.

We'd love to hear more of your insights regarding this so I recommend sending feedback to our product developers. This way, they can further review which parts of the program need enhancements and consider adding them to future updates. Please refer to the steps below:

1. Go to the Gear icon and click Feedback.
2. Provide your feedback or product suggestions.
3. Click Next to submit.

Moreover, if you're customer is not receiving your invoices, let's try a few steps to fix this. First, have them check their junk folder or spam folder. It's possible that the invoice was sent to that location.

If it's still missing, we'll have to configure your email setup. I'll guide you through the process.

1. From the Gear icon, select Account and settings.
2. Go to the Company tab.
3. Click the Contact info section.
4. Clear the Company email address field and enter the desired address, even if it shows the correct one. Make sure not to use any extra characters or spaces before, within, or after the address, you enter.
5. Click Save, and then Done.

Lastly, send yourself a test transaction. If you're getting the same result, we'll have to use a different email address with a different domain.

If the issue persists, your customers may need to perform some steps as well. Their email providers may have blocked our address. You can ask your customer to configure their server.

Here's an article for your guide: Troubleshoot if customers aren't receiving emails from QuickBooks Online. You may follow Solution 3.

If you want to let your customers pay the invoice online, you can send an online invoice. Here is a reference for more information: Take and process payments with QuickBooks Payments.

Let me know if you have other questions about invoices in QuickBooks Online. I'll be around to help. Take care and have a good one!