We used Mears in August. I would give them a call and they will give you a price. There were two ways for us to do it...to take a shuttle which was a certain amount per person round trip (for some reason I think it was $33 roundtrip per adult, cheaper per child) and or you could get their private van service. The private van service worked out better for us.
We didn't go from Orlando Amtrak to WDW, we took Orlando Amtrak to a hotel near Universal and it was $43 one way for 6 people, tip included. We took the Mears shuttle from All Star Music to the Amtrak Kissimmee station (it was cheaper for the shuttle to take us here than to take us to the Orlando station, and the Amtrak ticket was the same price). This cost us $53 one way for 6 of us. Our drivers were great, on time and very helpful with luggage! (We were two moms travelling and 4 kids and too much stuff, so we needed the help!)
When our train was delayed we called from the train and they adjusted their time. No worries! Have a great trip, and also, you may get a lot of good tips from the transportation board.