You can either use the javascript mode to insert the cancellation link, or you add the code - which is visible at the module configuration - into your template.
Both features does allow you to modify the output of the hint box.
Javascript Mode
Edit the file
cancelbox_template.php. The file can be found in the module directory.Manual Mode
You can copy & paste the template code from the text box at the module configuration and insert it in the view
viewinvoice.tpl. Modify the code as you want.