At the Porsche dealership I work at the SA's don't make a commission on the parts they sell. In the past, yes they did. I'm not sure how they are paid but the pay structure changed over the years. In the past every dept that upsold something would get a spiff. They don't any longer. The most common example is a new car is sold. The salesman and new owner walk up to parts and sell rubber floor mats and a cargo liner. Salesman gets his name on the invoice, a copy is made, and he gets a spiff. Now that doesn't happen for sales or service. But like everybody here that is a concern. Hard to ask the advisor if they get commission without insinuating that they might not be truthful when they come at you with the quote that the tech did for what your car or truck needs. Speaking of that, our techs at Porsche do quote brakes a lot. Saying that makes you think sure, easy money, why wouldn't they? But they also look at how the car is driven, mileage on it, track days, etc. If it's at 4mm now, by the time it's back for the next oil change it would be dangerously close to metal on metal. Assuming they got the full 10k on the LOF. So sometimes they quote brakes when the pads are just a bit more than min spec but there is a reason for it. Not to make an upsell but to let the advisor and customer discuss when they might bring the car back, or what are their plans for the car from now until the next oil change.
I went and talked to a service advisor at the selling dealer where my truck is coming. This individual didn't strike me as the kind of guy that got his hands dirty, ever. Wasn't impressed with his answer to my question that was really a test question. I think a service advisor is better if they are a car person. Otherwise you get double billed. They might not even know it or think about it. Say you are having front shocks, or front wheel bearings done and you need brakes too. Calipers have to come off anyway for either shocks or bearings to be done. The brake job should be a minor upsell but will you get hit full bore for both jobs even though they overlap?
Techs are paid on a scale though, I wouldn't say on how many parts they sell or quote but they do and can track how much money is generated on the upsells that the shop sells. More accurately the upsells that the advisor manages to sell. This is stuff that the customer did that they didn't come in for. But the more work the techs do, this sliding pay scale favors the tech that is experienced, works consistently, not always the fastest but doesn't BS with the other guys, and works with the parts dept to make sure that all the parts came in for the jobs he has lining up for the day.
Anyway, my point is that there are slimeballs out there. But like I read maybe on this forum here, the high percentage of trucks sold vs how many visit forums. Your usual forum member knows their way around their car or truck and will ask the right questions, know what to look for from a bad advisor. The techs, you never get to meet them although many of them, ours now, do videos showing you from their phone, your car and the shocks are leaking, the nail in the tire, brakes that are low, etc. This has led to more upsells and customers knowing they aren't getting lied to. Plus they could talk to our techs if they asked. Not all dealerships are like that.
And loaner cars, like MDJAK mentioned above, holy crap that whole thing is a nightmare in itself. If we don't get the parts from Porsche on time, that loaner has to stay out one more day but it was promised to somebody else. Parts catches a load of crap for something Fedex did, or didn't do, truck broke down, whatever. I feel bad for the advisors since they catch a lot of heat for stuff they aren't in control of.
I know I sound like I'm defending them, but they are people like us making a living but also making sure the vehicle is safe when it leaves the dealership. Truthfully we have a couple where I work that are a major pain in my ass in parts. Whole other story. But I don't think any of them are slime balls and do believe once they prove they are working for the customer, the repeat business is where it's at to keep their numbers up. Numbers, another story there. I'm sure we all have numbers involved with what we do in our professions.