When you first use TM3, invoices are not attached to emails that are sent out as standard. To add the relevant invoice to an email follow the steps below:
Navigate to Settings > Documents > Invoice > Invoice Cover Email > View.
This will then bring you to the Email Template Editor which allows you to edit the content of the email. To attach the invoice, click on the paper clip icon, attach the appropriate invoice and click 'Done.'
When the invoice has been successfully attached you will see the Invoice attachment on the template editor. Click 'Save' when you are happy with any changes made.