Hi, Scott. My experience has always been that it's cheaper to do train tickets when your there in that area. Have you been to Slovenia before?
Postojna is a pretty small town, and the caves are 1km outside down town. It IS good ways from the train station to the caves, and taxis may not necessarily be there - so I would try to plan that ahead of time. (Trust me - walking it is not fun. LOL. See car rental suggestion below.)
Postojna and Predjama (the Cave Castle) are the main sites. There are a couple nice restaurants, but the town is pretty small. There are, up in the hills some rock arches - like you'd see in Arches National Park, and there is a very large hill you can go up to for a beautiful view. If you'd like info on that, let me know, and I'll ask my friends in Postojna. The country around is also beautiful.
FYI - Predjama is pretty much a taxi trip (unless they've added a bus route). It's not expensive - but if you want to go see the arches, etc - it might be worth it to rent a car in Postojna or Ljubljana on your way down. Would get you to Skocjan too. The roads are wide, well marked, and easy to drive.
FYI2 - Going by train to Trieste, you actually have to take a bus from the border area to downtown, so it means moving bags again, etc. Just a heads up, cause it's kind of a pain. The bus ride is usually included in your train fare.
If you have any other questions, let me know. I've been in Postojna a lot, and have friends that live there.