I am traveling from US to Seoul to Shanghai and staying 7 days then back the US.

Paula, if your itinerary is Seoul - Shanghai - US, but without a landing in South Korea on your way back to US, you are eligible for 240 hour visa free transit and you do not need any kind of visa. However, if your itinerary is Seoul - Shanghai - Seoul (or anywhere in South Korea) - US, then you would not be eligible for 240 hour visa free transit scheme. In other words, you need an itinerary that has its immediate points before and after your mainland China visit in two different countries or eligible territories (eligible territories are Hong Kong, Taiwan and Macau). Have a nice visit to China.