In Gibraltar, the local taxi mafia operate the shuttle and charge £1 each way but you can walk it in 15mins. You need sterling cash in Gibraltar. You will not be going to Seville as the ship will not go up the river so you will dock in Cadiz, which is 80miles from Seville. However the dock in Cadiz is right in the town centre so no bus is needed. Not sure about Cannes but all the others a shuttle is needed. Not sure about current prices but two years ago on RCI we were charged $10 return per person chargeable to your ships account.
Last time we did Cannes we went by tender,beautiful place very up market and pricey.Civitavechia Shuttle service ok to walk round but this is main port for Rome you may think about a tour?Cagliari you should be able to walk into the city,not to sure about the rest.
To my knowledge the only shuttle you pay for is the one in gibralter it is approx £1 each one way.
This is really information but we visited Gib last year on a cruise ship and unlike 99% of the passengers did not go to see the monkeys but there are masses of dolphins in the area and that trip watching them taking photo's was extremely enjoyable and I am sure your 16 old son might enjoy as much as we did. On return to the ship they were all moaning about the monkeys pinching this that and the next thing and many hid in the cafe to get out of their way.
There is a shuttle needed in Livorno -as such a huge port to get to the gates. Seem to think it was $8pp last year. Cannes you are right in town. Civitavecchia no need to take RCI shuttle as the port lay on a free one (that RCI don't tell you about!) Everything bought on the ship is charged to your onboard account and everything is in US$ onboard.
Also, have you joined the Roll Call for this cruise - best way to make friends beforehand (esp with children) and you will find people to share trips with.