1. For adding a default message to all invoices for a selected client, click on the "Clients" tab.
2. Select the client, then click on "Client Details/Edit" in the right-side tabbed pane.
3. Go to the "Settings" tab, then the "Settings" sub-tab, and enter the desired text to be printed on the client invoices in the "Boilerplate for invoice of this client" field.