I'm not sure that all parts of that code are needed to delete the session. As a test of what is going on you can copy this code to a new page. Where this code was on the success page you should print out the session variable you are trying to retain. Finally you should try updating the new page to display the entire session with a var_dump($_SESSION) in it.
When you get to the success page you should see the value of this variable. Then when you visit the new page it should clear all session values except for this one. You should then be able to see what variables are set and their values.