How it could have transpired:

Owner: "That'll be $1.50"

You: "This must be your lucky day. I've saved you at least $75 because I'm only going to charge you $50 for the service call, including labor, so you can take the $1.50 out of what you owe me and give me the rest of the change."

However, I think you did the right thing. My best advise: Let it go. You did your good deed. Don't let others determine or control how you feel. You had a chance to do good, and that's on you. How he reacts to your goodness is on him. We often expect others to react like we would react and that leads to disappointment in others. If it is going to be something that bothers you, ask him about it and clear the air. Ask him why he charged you for the coke when you saved him over $125??? Otherwise, forget it and move on.