The shuttle departs from the Mecardo on I drive at about 8:30am and gets to Busch for about 10:00. I have used it numerous times even though I had a car as its much more relaxing especially on the way back after a day in BG. There isnt a choice of pick up times it just depends on the time of year. It is usually about 6pm but can be 7pm if the parks open later. As people have already said its free if you have the 5 park flexticket if not you have to pay $10 which you do when you get to the park. You need to pre book the number of people who are going which you can usually do at the hotel you are staying or there are plenty of flyers in the brochure racks with the telephone number.
Also if you are there when Howl O Scream is on in October they also did a late shuttle last year which got you there for when the park reopened (about 7:30pm ish) and pick up was at midnight.