Depending on where you stay in Yisan, a free airport shuttle could include a two-way trip to and from the airport, at certain hours of the day, or it could be just a one-way trip to the airport.
How often will the shuttle run?
It depends on where you choose to stay in Yisan. Some hotels have shuttles that run at regular intervals day and night; others require advance reservations for custom departures and pick-ups. Check the hotel's policy on Expedia before booking your stay.