Buying reserve tickets. Train station or online?
The only reason to buy on line is to take advantage of any discount tickets that still might be available. Since you are traveling in two days, I just buy them when you get to the station.
At this time. there are only base fares available for the 16th at 57 EUR 2nd class on Trenitalia. You might consider Italo at 52.90 EUR.
Both Trenitalia and .Italo run fast trains on this route and both have on-line websites and apps to allow ticket buying.
https://www.trenitalia.com/en.html
https://www.italotreno.com/en
For the Trenitalia site use the Italian town names for best results - Firenze and Venezia.
The trains are equivalent in services and times so it's up to you which works better for your time and/or price.
You are paying full price at this point and you can buy tickets at the station but I don't see any reason not to choose a time that works for you via the app or the website and then go to the station on your schedule.
=Tod