We have used Mears private car service for DDs last 2 trips at 34 and 40 months as well as a cruise we took just this summer when she was just over 5, and plan to do so again next March when she will be 2 months shy of turning 6. You can request a car seat during booking and DW and I love that it is a private car so there is no waiting around for anybody else. Once we have our luggage we are good to go. The very first time we used them our flight was delayed and the drive was excellent about communicating with us via text, confirming the delay and reassuring us that he would be there at our new arrival time.
It is a pricier service, relatively speaking, but not having to worry about waiting for anybody else plus the gratuity being included in the cost and the easy availability of a car seat as long as we request it when booking has us more than happy to pay that cost. We did take DD's car seat when she was 10 months old because we bought her a seat on the plane even though we didn't have to. We loved the peace of mind of knowing that she had her own secure plane seat but then having the car seat with us the rest of the trip was a big hassle.