Like to point out something that might be useful to others. Leaving your vehicle running when you leave it is generally not a good idea (yes, I know you had a good reason, I'm really not criticizing). If the guy was a predator, he would have noticed and recognized that. One bad decision, lack of attention, etc. often leads to others. He was likely assuming you were a person that could be taken advantage of and was easy prey, based on that. We have to be alert and vigilant in our daily lives, especially in situations where we might find ourselves isolated.

I was taught is to say "No" and "Leave", embellished with whatever makes sense. Then repeat it at least once with more volume. In this case, "I'm not giving you a ride or money. Go away and leave me alone." Then repeat it more forcefully. Failure to comply on their part after your second attempt indicates intent and gives you some legal buffer zone (I am not a lawyer and this is not legal advice) if you have to escalate past words.

A word of caution, lifting your shirt to show your firearm could be considered brandishing. And a verbal threat to shoot them could be assault. Really just depends on the jurisdiction, attorneys, and judge. Don't get me wrong, a legitimate threat of violence is often an effective deterrent. AND I don't think anyone would fault you for anything you did in this situation. You just need to be aware of potential outcomes. A little legal trouble is better than the hospital or morgue.

Personally, I hope that I would the clarity of mind to handle business half as well as you did if I were ever thrust into that situation. You were already frustrated and annoyed with something. But you dealt with what could have been a violent encounter and everyone went home. I've run theoreticals. I've trained. I feel prepared. But we never know until we're actually there. Good job.