How do I get the custom fields to be displayed in the emails sent to my customers?

You can insert custom fields in both the subject and body of your Invoice notification template. Click on the Gear icon on the top right corner of your screen and navigate to Email Templates -> Invoice Notification -> Show Mail Content.

You may then click on the Insert Placeholders drop down to add the respective placeholders.

P.S: If the value of the custom field is empty, it won’t be displayed in the email even if the placeholder is present in the template.