Reviews please.

Get Your Guide is a 3rd party aggregator. They don't operate tours but operate as a clearing house for companies which do. They're essentially a middle man. A tour that you're looking at on their website should display the name of the actual company providing the excursion. It should appear in small type right under the headline; look for "Activity Provider: (name of company)"

Example:
https://www.getyourguide.com/piran-l1233/skocjan-unesco-caves-and-piran-t201772/?ranking_uuid=5f2930c9-ccc3-4b9e-97bc-fd586b2b16c5

Yep, right under the headline is, "Activity provider: Gray Line Slovenia Explorer."
It's not unusual for tours booked directly with the provider - in this case, Grey Line Slovenia Explorer - to cost a bit less. If something goes wrong, it can also be best to be working directly with the provider versus a middle man so as not to possibly to end up in the middle of a finger-pointing dispute.

I've not used Gray Line Slovenia Explorer myself but here are some reviews for starters.
https://www.tripadvisor.com/Attraction_Review-g274873-d2639535-Reviews-Gray_Line_Slovenia_Explorer-Ljubljana_Upper_Carniola_Region.html

And a couple of mentions on a previous RS forum thread:
https://community.ricksteves.com/travel-forum/slovenia/slovenia-explorer-vs-do-it-yourself

Kranjska Gora is a ski area on the north side of the Vrsic Pass. It has a small village
as well as a ski jump used for competitions. Aside from that, I'm not sure what else
there is to see there. If you get to go over the pass as part of the trip, that might be
worth it. Piran seems an awfully long way from the other two places, I wonder how
much time you'd spend in the van/car.

Skocjan Caves requires a guided tour provided by the caves, so any guided tour seems
like it would be mostly transportation to/from Ljubljana. In which case I'd consider just
renting a car, it's an easy drive. That would also give you the opportunity to go to
Postojna Caves, in case you want a different type of cave experience.

My plans were such I didn’t actually use them, but Slovenia Explorer was so friendly, helpful and responsive that I recommend them. As Kate said above, Get Your Guide is basically a booking site. That doesn’t mean it’s automatically bad though, just a little more opaque.

Thank you all for your help. It means a lot.

Kate, so appreciate your obvious experience.