Though purchase in viator could be cheaper, you dont receive a confirmation number, which is essential to follow up the van progress specially in the return trip. it is necessary to call supershuttle to book the return trip as well, since this is not an automatic process. Supershuttle website doesnt have any kind of online support system, email could take as long as 48 h to be replied, the only way to contact them is trough phone, which can be very problematic for international travelers which might have problems finding a phone to call and with the spoken language. I wouldn't book it again. Not using viator anyway, too stresfull. The only thing you got is a so called booking reference which is as good as nothing for online followups.
This was our worst experience with a shuttle company on our recent trip to the States. I am giving 2 stars because we did get to our hotel eventually but there was a lot of waiting around at the airport and the driver was very off hand. He took our roundtrip voucher which was the only record we had of our booking and told us we should have 2 vouchers then when we questioned what we would use for our return trip he told us just to ring the office. When we rang the office to confirm our return trip and discuss our lack of voucher they had no record of us and they said we would have to pay if we couldn't produce the voucher. After 3 calls they agreed to book a pick up but we had to get another copy of our voucher and they made no effort to trace our original driver. In the end we decided to make alternative arrangements. On return we were refunded.