Level 1

Using custom fields in custom email template

I have created custom invoices and custom email templated in QBDT. I have named one email template 'Balance Due' so I can sent customers updated balance notices. I have added fields 'Payments/Credits' and 'Balance Due' fields to my custom invoices. I need to add these custom fields to my email template to show the customer their payments and balance. 

I have searched numerous threads and haven't found the answer. Please advise!