You don't reserve a shuttle in Vegas - all "rerserving" does is prepay your fare. Note that a cab to most strip resorts will be $10-15 plus tip. If you have more than one person, or just value your time in Veags they are probably a better deal. Also, note that cabs are almost always a better deal for going back to the airport - the shuttles are not always good about picking up people at the scheduled time.
Note - Vegas cab drivers are notorious for long-hauling customers from the airport to the strip (that's the only way a cab from the aiport to the Mandalay - which is across the street! - can cost $20+). Be sure to tell the driver "no tunnel" if taking a cab to/from the airport.