First off, it is never wise to purchase tickets close to arrival. Too many variables you cannot control. Lets walk through it --
Arrive 8.25. Assuming your are on time. Allow a min of hour and a half to exit plane, clear immigrations and to the train stop.
9.00 AM - another hour to Centrale train station because you just missed the train and you have to wait 30 minutes for the next train. The trains run every 30 minutes so not sure why your are saying 9.43 and 10.43. And no discount for on-line purchase. Assume you are referencing your train to Florence.
10.00 AM. Probably need at least 30 minutes - rest room stop, maybe grab a snack, and find platform for 10.43.
It is OK, you made the 10.43, most likely would have missed the 9.43.
Now for variables your cannot control - immigrations is dice roll depending on the number of officers on duty and the number planes arriving at the same time. Last May in Rome it took over two hours just to clear immigrations and other times have been through immigrations in 20 minutes. If you plane is 30 minutes late then everything pushes back. If for some reason the train to Milan is not running, then the bus takes an hour.
This is the time to buy your ticket to Florence when you buy your train ticket to Centrale. Then you know you exact arrival at Centrale. You will pay base fare but that is very reasonable. Anything else is a gamble.