D.P.
Who is telling you that you need a carseat for your 18-mo-old? The shuttle company? The bus line? Someone else? If it's someone with an opinion but no knowledge, then disregard. If it's the shuttle company, ask if they have a carseat you can use (we have taken the airport shuttle from home to the airport, and they provide a carseat. We have taken the shuttle from the leave-your-car parking lot to the airport, and there's no carseat needed/required, though I wish there was!).
Most buses (bus tours) do not have seat belts and I'll bet they also don't have the LATCH system in their seats - how would you install a carseat there? Small children do not use carseats riding on city bus, right? Not sure how a bus tour would be that different.
Get on the phone and call - if the first person you talk to says you need a carseat for this or that, but can't tell you how it will be installed, then ask for a supervisor for the straight answer.