When generating an invoice using the QuickBooks API, I specify the customer ID.
When viewing that invoice within QuickBooks, the email field is not populated, even though the customer has an associated email address.
Thanks for contacting us here in the Intuit Community forum, @jeffpowell.
Let's get you to the right support team who currently handles QBO API.
Right now, app integration is handled by our Intuit Developer Team. They can help populate the email address on the invoice.
Please know I'm always around to help with any QuickBooks Online issues you may be having. Don't hesitate to leave a comment below or post again.
Thank you and have a great day ahead.