I have googled and found a bus company (drd.si) that offers a daily bus service from Venice to Ljubljana. Has anyone tried this company?
It is reliable and recommended in Rick's guidebook. Another popular option is GoOpti shuttle service.
It goes from Marco Polo Airport
We took it to Ljubljana and it was fine and quicker and cheaper than using the train. It arrives and leaves from the train station in Ljubljana.
"Reliable" is a bit too strong of a description of DRD. I know someone - an experienced traveler - who got stuck at Marco Polo airport because the DRD "bus" never showed up.
I've used DRD twice. A few years ago, I took DRD using a real bus with "DRD" written on the side from Mestre to Ljubljana. In May, I took their shuttle from Macro Polo airport to Ljubljana. It wasn't a "bus" this time - it was an unmarked van. I was waiting in the bus arrival area at the airport. You're standing there looking at each bus and van - is THIS one DRD? No - this one? Then a guy walks over with a sign that says "DRD" - are we waiting for it? (Lots of other companies had buses pull in - even though there is construction at the airport, I got the idea that DRD was simply trying to save money by not having to pay to enter the airport parking area.) I was waiting with two Aussies who had bought tickets on it. I had no ticket but had emailed DRD. They responded only about an hour before it arrived, saying they had an empty seat. I'm not sure if they would have come at all had the Aussies not had tickets, and I'm not sure if they'd even have responded to me had they had no empty seat.
The thing to keep in mind about DRD is that they aren't a conventional bus company. You CAN'T simply show up at a stop and wait for their "bus." There is no "DRD bus sign" to find in Venice - you have to look for their vehicle, or at least look for the driver holding a DRD sign. You MUST contact them in advance - email them, call them, even buy a ticket on their website. Having a phone you can use to call them near the time of arrival to confirm is highly recommended. If you don't, they may not show up and you may be stuck like my friend was.
Next time I'll probably use GoOpti.