Include custom message in Estimate and Invoice email templates.
To have this custom message displayed in your email, you will need to include a simple piece of code in the email template, which can be accessed by going to Admin > PDF/Email Templates > Email Templates.
For the Invoice email template, add: {{custom_invoice_message}}
And for the Estimate email template, add: {{custom_estimate_message}}
The location of the custom message code in the email template will be where your custom message will display when the email is sent to your customer.
For more information about editing email templates, please see the following article: http://feedback.repairshopr.com/knowledgebase/articles/225759-how-do-i-customize-my-email-template