Online Payment for mxp airport to Centrale

Yes it is legit. I see they have a promotion for 8 EUR. Can only be used on the day you specify for travel, and only by passengers named on the ticket, but otherwise good on any bus that day. It will deliver you outside the station next to it, then go inside and up the moving ramps to track level. They go every 20 minutes and take about 50 minutes, but can be effected by traffic.
If you take the Malpensa Express train instead, it is 12 euro and it will deliver you inside the station at track level. They go every 30 minutes and also take about 50 minutes, but will not be effected by traffic. They can also be bought on-line at https://www.trenord.it/EN/ or http://www.malpensaexpress.it/en/. There is a 3 hour window when they are valid from the time and date you specify on the ticket.

I just did this in May, I had pre-bought train tickets and had to get to Milano Central station, our plane was to arrive at 8:30 am and I had 12:10 train tickets to Monterosso, well as luck would have it we were 2 hours late leaving JFK due to a mechanical problem and I figured that I would just have to buy new tickets for a later train. The pilot made up 45 min in the air and we were allowed to deplane with the first couple of people, since we are true Rick Steves followers we only had our carry on's so we ran through to the immigration desks and were the first people there, got our passports stamped and grabbed a taxi and made it to the train staion with 10 minutes to spare!! Needless to say, we made our train, but I learned a lesson, it would probably been better to just have bought the tickets when we got to the station instead of pre-buying online. The Malpensa Express only runs every 30 min - 45 min so if your schedule is tight and you just miss it you would be better off grabbing a taxi. If Milan is your final destination then you can relax and take a taxi if you don't want to wait for the Express train, or if money is an issue then the Express is worth waiting for assuming that your hotel is close to the train station or within walking/metro distance.

However, I see on the malpensaexpress train site that they are expecting a strike beginning 21:00 Sept 29 to 21:00 Sept 30, so the bus seems like a good idea. Check to see if that will effect other train plans.