American- Flying direct Shanghai. Stay for three days. Do I need a visa?

Hello- I am an American. Have always wanted to visit Shanghai. Got a great deal on a flight and booked the ticket to leave in 13 days. Didn't occur to me that might need an entry permit. Staying four nights and then flying back to the U.S. do I need an entry permit and if so what are my best option for getting one so quickly? Thank you.