Train from Milanto Venice

It depends if you prefer maximum flexibility of choice, although many seats will be gone there will be some, and don't mind paying the maximum (full) price,

or if you prefer the cheapest price and are willing to commit to a particular train at a particular time at the risk of losing your money if you miss it, and then have to buy a new full priced one.

Money or flexibility.

Are you arriving in Milan and traveling to Venice the same day? If so, no. The savings are not big enough to justify that you will lose your ticket if the flight is delayed and you miss the scheduled train.
If you are going to be in Milan for a while (at least one day) and you like saving money, buy a nonrefundable ticket up to 4 months in advance. Print-at-home tickets.
Full fare tickets are 38.50 EUR. As low as 9 EUR 4 months ahead.

If you are overnighting in Milan, I'd buy the ticket on arrival at the train station or swing back later. Lines for tickets can be long and can cause you to miss the next train - yes, even at the ticket machines. I have been stuck behind tourists who took forever because they didn't know how to use the machine and didn't know how to ask anyone for help. I've also been stuck behind people buying multiple tickets for subsequent days, checking schedules, prices and the devil only knows what else.

BTW, all ticket machines accept credit cards but only some accept cash. On the top, they will have a € sign if they take cash. U.S. cards do not always work at the machines. If you are going straight to Venice from Malpensa, you can buy your Malpensa Express tickets to Milan from agents or machines, but you can only buy Trenitalia tickets at the machines outside the Express office. When I was last there (1.5 yrs ago), none of them accepted cash and I had to buy my Venice ticket at Milano Centrale. Be sure to take the Express train to the same station as the Venice train leaves from.

Thank you all for responding. I will be arriving in Milan at noon. I think I'll go with flexibility. The train to Venice takes only about 2-1/2 hours so I think I will just buy the tickets at the station.

Buy your ticket at the airport at the same time you buy your ticket to Milan. When you buy the Venice ticket they should be able to tell you the departing platform. Do check it when you get to Milan Centrale because platform can change within the last 15, 20 minutes.

Not sure if you can do that without a chip&PIN card. Chani just reported that the Trenord ticket window (operator of the Malpensa Express trains) does not sell Trenitalia tickets. There is a Trenitalia kiosk there, and I have used it in the past, but with a chip&PIN. But by all means ask when you are there. It saves a lot of time to get the ticket there.