How to send an invoice to multiple emails addresses?

Hello, I have tried building this workflow so that when our team is ready to send an invoice, it will be sent to the primary email on the customer record and the string of additional emails on the customer record. I’ve followed these instructions exactly as presented: Set Email Address on To Be E-mailed textbox from Customer record to the Invoice(transaction) Record via Workflow (custhelp.com)

The error I am experiencing is that all the customer fields including no custom fields are showing as an option to be selected from (see image below). The custom field that was created to be mapped here is called "Multiple Customer to be emailed" but it is not showing from the dropdown list. Many fields are not showing here in the dropdown.