When emailing invoices, how can we include current invoice amount and past due balance in message body. what replacement tokens are available for email templates?

I got your back, @accounting491.

You can edit the message body in the email to add the current invoice amount and past due balance. Let me guide you on how to:

1. Click the Gear icon.
2. Choose Custom form styles. 
3. Look for the invoice template you use and tick Edit or click the New Style button to create a new template. 
4. Select the Emails tab. 
5. Under the Standard email section, add the Current Invoice Amount and Past Due Balance on the Message to customer box. 
6. Once finished, hit Done. 

Next, when you're ready to send the invoice, just enter the corresponding value for Current Invoice Amount and Past Due Balance before clicking the Send and close button. 

Just in case you want to view all of your customer's invoices and payments received, you can run the Invoices and Received Payments report. Here's how:

1. Go to the Reports menu.
2. From the search field, type Invoices and Received Payments.
3. Click the report to open. You can also modify the report details by selecting  the Customise button. 

Let me know how this goes and post a reply below if you have further questions in customizing your email message body.