getting to Venezia Mestre train station

You can either take a train from the Santa Lucia station or a bus from the Piazzale Roma. In either case, they are very frequent and inexpensive.

The Mestre train station is just across the bridge from Venice Island. Mestre = mainland, St Lucia = Venice Island. I'm curious, are these tickets from RailEurope?

Buying a ticket from Mestre instead of Santa Lucia was probably a mistake you made on the website.
Most trains from Venice start at Santa Lucia (which is a dead-end station on the islands) and stop at Mestre on the mainland.
Easiest way is to find out when your train leaves Santa Lucia (about 5 minutes before Mestre), and buy a ticket from Santa Lucia to Mestre. Then you don't have to change trains.

Some of the trains to Florence originate in Trieste, so they don't stop at Santa Lucia. What time is your train, or do you have the train number? Its easy to look up at www.bahn.com.
There are Regionale trains going from Santa Lucia to Mestre almost every 10 minutes. Tickets cost 1.25 EUR.
If it is a Freccia Train (just about guaranteed) and it does originate in Santa Lucia, the ticket will be a bit more.
Edit-The Trenitalia site does not offer tickets between SL and Mestre on Freccia trains. It is 10 EUR on IC and EC trains, but they will not be going to Florence. So I'd say suck it up and get the cheap Regionale ticket and make the change.

Yes. If you have to get off at Mestre, just buy a ticket on the next train to SLucia. A couple of years ago it was only 2 euro. Probably 3 now. Or ask the conductor if you can just stay on till SLucia. He might say sure and not charge you anything. It is less than 5 minutes.

I think the OP is going the other way.

Yes it was a mistake on our part. And yes they are from Rail Europe. I noticed the train does originate at Santa Lucia. However, it is an Italo train and I can't seem to buy a ticket from them to go from Santa Lucia to Mestre. Live and learn!

You probably cannot. Just buy a ticket (2 euro probability) from S Lucia to Mestre and then catch your train. Italo is new, somewhat private train line running on Trenitalia tracks. A little competition for Trenitalia. So they may not go to S Lucia. You can also take a taxi from S Lucia to Mestre. Very close so it is not a big problem. Just adds a little more time to your schedule.

If it starts in S Lucia I would get on, just look for a conductor and explain the situation, offer to pay. My guess is that you will not see conductor until after Mestra simply because of the time.

Just took a quick look at Italo's own website, and it appears the only option when booking tickets for Venezia is all stations ("tutte"), you can't even choose Santa Lucia or Mestre (at least I couldn't make it differentiate between the two.) They do offer options for Roma, so it doesn't appear to be just some website fluke. As someone else said, see what time the train is set to depart Santa Lucia, board, and find the conductor right away. It doesn't appear there is any price difference.

Yes, nobody is going to jump on the train at SL and take your reserved seat then get off at Mestre.
There should be train personel standing on the platform to ask, earlier the better, maybe even the day before. I don't recall if Italo has a ticket window at the station, but I never looked. If they do, ask them.

The problem with buying tickets at RailEurope for specific stations is that you cannot request a specific station in your search. All you can choose is Venice - and RE will list all of the trains in both Mestre and Santa Lucia and rail traveler newbies are sometimes unaware that it does make a difference so they just choose the one with the appropriate time of travel. This is especially true with Florence also.

RE has several problems - this is only one of them.