No. Your e-ticket will be automatically e-mailed to you after you complete your booking online. All you need to do is to bring a valid picture ID along with a printout of the e-ticket you receive to board the scheduled bus.
You may bring a car seat if necessary, although it is not a requirement. However, every person occupying a seat needs to have a ticket, even infants.
For most of the tours, children under 2 years who sit on your lap are not required to have a ticket, unless otherwise indicated on the tour page.