Do I need a visa or can I get free transit?

I am US citizen with a passport. I am cruising in September to Japan from Vancouver BC. My Japan cruise will end in Shanghai PRC. We then are flying out of Shanghai (PVG) airport back to Vancouver BC. The cruise ship line "Celebrity" said we need a Chinese entry permit. Is this the case or can I do the free transit? AND how would I go about doing that? I greatly appreciate any help.