This article also answers

What is the send email button on the invoice?

Resend invoice

How do I resend invoices?

Solution

To send invoice emails automatically
- Enable invoice receipt email
To do this, navigate to Settings > Configure Chargebee > Email Notifications > Enable "Invoice Receipt" email notification

In case you need to manually send the invoice, you have an option ‘Send Email’ from Invoice to resend the email. Navigate to Invoice Index Page >  Select Invoice > Send Email

In case the user request to forward the email to a different email address Chargebee provides you an option to forward it. Navigate to Logs > Email Logs > Select Email > Forward