Is it really true that in the U.S. you have to go through customs, even if you are just connecting to another country (say, Canada?). I've never been to another country that required passage through customs. TPE is not like that, as you might expect.

I didn't know about the free tour of Taipei. But I do know that it is a long trip to the city (45-60 minutes?) and there is no rail system (just bus and taxi).