I agree with everything you wrote. Find nothing to contest. Just FYI; 15$ is roughly 100 kuna and this buys ONE of the listed (so it's either or):
3.5 pounds of prime turkey breast no skin no bone
2 pounds veal cutlet top quality
15 average loafs of bread
20 carboard packs of milk (one liter, good brand)
Monthly mobile phone fee (average package, nothing packed with options and features)
10 liters (2,2 gallons) of gas
I think you SHOULD have charged finding the problem. My guy was in the vicinity and in our terms that's just a few blocks away. He did invest his time, but found nothing. It's a border case at best. Charging failure is a tricky one.
But, as
@pewternhrata wrote, you can't go driving around all day giving people free diagnostics and then let them call someone cheaper who might even rely on your diagnostics conveyed by the one who has the problem.
Still, in my case, there was very little to pay.
For some reason those insect nets, especially ones that are built in the window, cost an arm and a leg. That's why I was reluctant to say the least. I don't want to tell myself I know it all. That's usually the downfall. Even if I see where the problem is, I still try to spin a few different scenarios in my head and predict how my meddling might make it even worse.
I fix most of the stuff around my apartment, but I'm always this way; measure five times, cut once!