Thank you guys for the welcome and replays. I did try another day which gave me the option to select more than one passenger. The MINI fare available for Venezia to Firenze was 9 euros (IC train) and 34 euros (ES train). The IC is only 54 minutes slower than the ES train (2.57 hrs rather than 2.03 hrs). I am not sure if saving 54 minutes is worth the extra 25 euro for the faster train. I am still working on my dates and itinerary and I will come back and let you all know if was able to book online with US CC. I was told by a friend that Capital One Visa worked fine on Tranitalia website. We shall see. Thanks again for the prompt and kind replays.