Honda put an air filter on the passenger seat of my car once then charged me for it. I have a washable K&N FFS

You're right, it's trying it on. You would know if they'd used some I think

 

Many moons ago BMW did the washer fluid one on me.

I knew it was full & queried it.

They removed it from the bill.

I got in the car & there wash a new bottle of washer fluid on the passenger seat.

 

Times are hard and these dealers will squeeze for as much as possible.

I still remember some a gent (Marcus IIRC) telling me that Honda had charged him for his tappets adjustment.

Being a generous person, I told him i'd have a check see if they had. They hadn't

An old boss of mine caught out the Land Rover dealership in Chester, who he had suspected for a while, by marking things like the air filter, and placing blobs of sealent on parts that would have to be opened to service. He was charged for a number of things that simply weren't performed. The dealership blamed it on a 17yr old YTS lad working for them...

 

I wonder if my clients would be happy with a charge on their invoices for the disposal of the 'fluid' created by the guys in our studio who work on their projects...

The waste oil from garages is sold for a tidy-sum too of course.
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We charge our customers for disopsal of removed service items and liquids. We just don't itemise it to prevent arguments.
It is common practise now as they have a duty of car to dispose of it correctly. You need a licence to transport this sort of waste now, they will be paying to have the oil etc removed. (same tyres now) The waste company is making the money.
The washer fluid is also common practise they all try this on. Ask for the scope of the service and specifically point out what they should not do when you drop the car off. I do it every time -oil for my current care is £210 from the dealer and £114 from Opie, I never let them do the screen wash, wipers, wash it or valet it.

 

I had a car serviced once and the guy said "I stuck a set of new wipers on it, only 5 pounds" I had only just put some high quality ones on and the ones he charged me for were rubbish.

I made him get mine out of the bin, swap them and give me my money back.