On an upcoming trip to the USA, I've booked a ~$300 shuttle to take us from the airport to a city two hours away. I've prepaid it, if that has any bearing.
A few days later I'm using the same shuttle service for a second leg, roughly the same price and distance. Booked but not paid yet.
As always when travelling to the US, I'm uncertain when and how much to tip. Would I be expected to tip around 20% per leg = $60 each time? Upon arrival?