From the Site Invoice Settings (Admin Interface -> Settings -> Site Settings -> Site Invoice Settings) menu set:

Invoice Address

Enter the address that will appear on site invoices.

Invoice Header

Add a custom html header for site invoices.

Note: Leave empty to display the site logo.

Invoice Footer

Add a custom html footer for site invoices.

Front End

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.