I remember when, as a new mother, I was about a dollar short in the checkout at the supermarket and told the cashier I would not take one item. The man behind me quickly gave the cashier the dollar, I thanked him profusely and felt so good about other people for quite a while! I have done the same a couple of times; the last time a woman refused to take $2, and I felt so sorry for her -- she lost the opportunity to thank someone and feel grateful, and I lost the chance to help (but I will still keep trying)