From the Site Invoice Settings (Admin Interface -> Settings -> Site Settings -> Site Invoice Settings) menu set:
Enter the address that will appear on site invoices.
Add a custom html header for site invoices.
Note: Leave empty to display the site logo.
Add a custom html footer for site invoices.
From the the Additional Information (Home -> Members Area -> My Account -> Account Settings) menu complete the:
Sales Invoices Custom Content
Add custom content to sale invoices.