It has a lot to do with developing patience, not with the check-out person so much, but with your own pain that arises, the rawness and the vulnerability, and sending some kind of warmth and love to that rawness and soreness. I think that's how we have to practice.
Be grateful to everyone" is about making peace with the aspects of ourselves that we have rejected... If we were to make a list of people we don't like - people we find obnoxious, threatening, or worthy of contempt - we would discover much about those aspects of ourselves that we can't face... other people trigger the karma that we haven't worked out.
Sticking with uncertainty is how we learn to relax in the midst of chaos, how we learn to be cool when the ground beneath us suddenly disappears. We can bring ourselves back to the spiritual path countless times every day simply by exercising our willingness to rest in the uncertainty of the present moment—over and over again.
If your mind is expansive and unfettered, you will find yourself in a more accommodating world, a place that's endlessly interesting and alive. That quality isn't inherent in the place but in your state of mind.