72 hour permit to enter Shanghai

I am planning on entering Shanghai using a 72 hour permit. We are flying from Tokyo to Shanghai and then back to the US. We will be in Shanghai for less than 72 hours.

I have printed airline ticket for the return trip home as proof that we will only be staying for less than 72 hours. I also have a valid U.S. passport.

Do I need any additional documentation? Is there any additional information I need to know. Thank you.