What's the difference between someone who donates because it makes them feel good (to have a positive impact on the world) and someone who does it because they want to have a positive impact on the world?

There isn't one. It's the same thing.

All choices can be traced back to a "selfish" reason. To deny that is to suggest that there is something other than our self that determines if we are a good person or not.

This comes from a lot of reflection on ‘selfish altruism’. You might think it’s divorced from reality but I’ve had to argue for altruism existing far too many times in England to not think about it more. It’s probably the cause of donating far more often than you think, just people rarely admit it.