I can show you two ways to do this which worked for me as well, danieljlmiller.

At this time, adding the customer's email address and phone number on the invoice to be printed or sent is not available. The first workaround is to add these details on the Billing address section. This way, they'll be displayed when creating, printing and sending the invoice.

Here's how:

1. Click Sales on the left panel.
2. Select Customers.
3. Click the customer's name on the list.
4. Click the Edit icon in the upper right-hand corner.
5. Go to the Address tab at the bottom, and then add the email and phone in the Billing address field.
6. Click OK.

The second one is to set up two custom fields for the email and phone. Then, manually enter the details when creating the invoice.

Here's how:

1. Click the Gear icon.
2. Under Your Company, select Account and Settings.
3. Go to the Sales tab on the left.
4. Click the Sales form content section to open it.
5. Below the Custom fields section, check two boxes and name them Email and Phone.
6. Click Save, and then Done.

I'd encourage you to send a feature request to have this option added in an update soon.

Here's how:

1. Click the Gear icon, and then select Feedback.
2. Enter your request and click Next.
3. Click Send message.

I've added screenshots too.

Please let me know if you need anything else.