I agree with Hank. Upon previous advice from him, when we went to Florence, we got a taxi and did not wait for the shuttle. We DID take the shuttle back to the ship though.
Like the OP, we found another couple wanting to get to the train station and shared a taxi. it makes the cost easier to take if you split it 4 ways.
You want to get to Florence as soon as you can to beat the cruise ship crowds.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013