I am upset with them

Let me first say that I have taken my car to them previously and I have been satisfied with the work done. I went to Advanced Auto Pro to have a flasher replaced back in December. The turn signals were not working. They replaced the flasher and the turn signals again worked. What I started to notice was that occasionally the instrument panel did not work. This seemed to be erratic and I continued to drive the car with the instrument panel not working some of the time. I checked on the web and one of the suggestions was there might be a loose wire. I finally decided to take it back to them. They recommended that I get a new or used part that would fix the problem. It was to take a day for the part to arrive so I took the car and planned to take it back once the part arrived. The problem did not reappear and so I decided to drive it for a while. When the problem did not occur again ( I have driven the car something like 250 miles to this point and it has not reoccurred) I went in to pay the bill. They had to return the part and there was a restocking fee. I understood that it was reasonable for me to pay this. However they also charged me for the work investigating the problem. Since the original problem apparently was a loose wire which was caused by the mechanic when he replaced the flasher, I believe it was not appropriate to charge me for the work done. This charge was approximately $95. I did ask about this but they seemed unwilling to remove that charge. I paid the bill and now I am upset with them since to keep a customer happy they refused to consider removing that charge.