That ain't karma. The real way karma works is that deeds in one life get justice in the next. In the popular connotation, karma would have given you a free day-old donut on top of your purchases.

The clerk is the person in this story with good karma. They did something nice for you which could have resulted in a loss for them. All you did was what you're supposed to do, exchange money for goods. If you're thinking that is karma, that means the thought of not paying entered your head. That's what the burn was for.

You are confused. The clerk who let you do that earned the good Karma, not you. They did you a favor. While it was decent of you to come back and pay, that was the expectation not something out the ordinary. Sometimes shit just happens…