72 Hour Visa Free Transit Question

I am travelling from the USA to Shanghai in October. I will be flying from Dallas to Hong Kong (couple hour layover) then arriving in Shanghai. I will be staying in Shanghai for 3 days (under 72 hours) to explore the city and then have another confirmed ticket on to UAE (Abu Dhabi). I have a USA passport. I just want to make sure I qualify for the free transit. I should just let the Airline know when I check in for my flight in the USA?