I think I qualify for 144-hour visa but what if I'm wrong and they disagree?

The rule is quite clear: duration of stay starts from the day after the day of arrival. You do not need to worry about getting refused entry under that if your routing qualifies (i.e., Shanghai will be your only stop in Mainland China).

In the highly unlikely event of being refused entry, immigration will escort you to a ticketing counter and you will be allowed to purchase an outbound ticket to your destination of choice. Again this is all highly unlikely if everything is in order.

Charlotte, could you present your exact days and hours of arrival/departure to/from Shanghai.

Thanks for your help. Arr July 27 around 7pm local time and leaving Aug 2 around 4pm. What if I'm delayed by a flight cancellation or bad weather? Will I be required to get a Visa for the extra 1-2 days I'm stuck there? Thanks again.

Your dates and times are definitely within the frames for 144 hours visa free transit, so you should not worry about it any more. In case of adverse weather conditions or affected by flight cancellations, this is treated as something outside of your abilities and you should also know that in that case you will not be treated badly in any way, but you will be normally allowed to stay until the situation is resolved. Everything that matters is your scheduled arrival and your scheduled departure and that's it.

Zupan

If a flight is delayed and you have to overstay your 144h, you have to contact the PSB immediatly to solve the problem.
They, for sure, will exceed the temporary permit but if you don't contact them, it can cause you some problems in bad case.

Of course... I just wanted to calm down the situation by confirming that any delay caused by force majeure is not a reason to think that with such a circumstance she/he has done an offense and will be treated in some bad way.

Thank you Zupan and everyone else