Everyone I know has gotten ripped off at the mechanics
Unfortunately, some car mechanics in Poland are a bit like most solicitors; they believe they have a God-given right to charge whatever they want for a crap service.
Here's a good one. A friend of mine recently heard a strange noise from her car. She took it to a mechanic, who shook his head and estimated, after making a phone call to his 'colleague', 3000 PLN to repair it. She said she'd have to stop using the car, save up some money and come back. A couple of days later she was passing a garage which she remembered was owned by the father of an old classmate and pulled in to get him to have a look. He reconnected a wire, didn't charge her a penny and the car is now fine.
I had some terrible cars in Poland back in the days when imported cars were prohibitively expensive and I had to buy junkers that I could afford, and I wasted a lot of money having them fixed (or Fixed) at backyard mechanic's. Even now, things are dodgy. I bought some summer tyres this year and the garage told me my steering arms would break in max. two months. I took the car to the mechanic I've used for several years (they don't sell tyres) and they said there was nothing wrong.
Find a reputable mechanic by talking to people, and stick with them. I must have been to a dozen places before I started using the one I've now used for about five years. They are decent guys who tell the truth, provide a full faktura and know that if they rip clients off, they won't come back.
Is it really any different in other countries?