Karma doesn't exist at a supernatural level, however, certainly can be examined at a level of simple cause and effect, what you do to others or your environment is going to effect you in some way. Your behavior will dictate how others interact with you.
I would also like to mention, you aren't being a nice person if you expect something good in return, which is the underlying expectation with karma.
I love this explanation and agree with you 100%. I hate when people are intentionally nice for selfish reasons, such as building up their own reputation or hoping for something in return. I believe an act of kindness should be done with no expectations other than the satisfactory feeling of helping another person in need. That should be a big enough reward in itself.