I took my car infor a service once, having replaced the metal caps for plastic ones. The car came back with the plastic ones gone and shiny metal ones back on, plus a bill for 4 x £21 for each of them. I told them to put the old ones back and they said they binned them, so I told the to F**K off and trawl through the bins to find them and I was knocking £50 off the bill for wasting my time. In the end they gave me the £50 off and found some silver looking plastic ones free of charge.
I just wish they would ask first before they just do things that are outside of the service.