I think you've gone through what is probably near the worst thing that could happen at a cash register due to being anxious or nervous. If you read over your post though, and look at the situation objectively, do you really think it is something to be scared of? You're not hurt, and having not done anything illegal, you never had anything to worry about legally. Some guy stared at you. It was uncomfortable and awkward, but that's the worst of it.
I deliver pizza's, and a lot of times my lack of social skills are very clear to them. It'll feel very awkward to me, and in the car I'll talk at myself at myself for a little while, running through it over and over. The next day I won't even remember it though. It's something I don't like to go through for sure, no question, but it's not going to stop my from doing it though. Awkwardness and discomfort is nothing to fear.
I can kind-of appreciate what you're getting at. When that staring incident that I detailed happened, a small part of my mind wanted to think, "O.k. - well, I've gotten the 'worst' scenario out of the way, and I survived. This should remove some of the fear of it happening in the future." Of course - the more powerful part of my mind had other ideas....it's just difficult for me to really embody that line-of-thinking. Some of this is due to the severity of my S.A.D. - where one's own mind can be one of her worst enemies.
Also - the stares themselves are torturous-enough - but there is more to the problem than the stares alone. It's not only the stares - but what the stares signify that is so difficult to bear. Staring is a bold, aggressive, invasive action; often, people are afraid to be caught staring at someone. You don't dare stare so rudely at someone whom you respect or fear. If someone dares to stare at you (if you are not extraordinarily beautiful/handsome, or if you are not famous) - it often means that they have (subconsciously) assessed you as a "weak", peculiar individual who doesn't merit the same level of respect as more "normal" people. They aren't granting you the same rights to privacy and dignity as they grant other people. It's a debasing act.
And the fact that these people are emboldened to stare at me - while I personally witness them NOT staring at other people in anywhere near the same manner - is quite upsetting. For one thing - see what I wrote above. For another - it confirms and reinforces those ugly, SAD-related thoughts that I struggle with concerning how embarrassingly "different", awkward, and conspicuous I am. I also have this intense dread of people noticing the outward signs of my mental struggles (i.e. SAD, Major Depression) - such an intense dread is not exactly uncommon among SAD sufferers - and the fact that I get stared at just a nightmarish self-fulfilling prophesy.
Also - I cannot just roll-over and accept the fact that - by staring at me and directing other actions toward me - they are letting me know that they are suspecting me of shoplifting. It's maddening and debasing - because I've NEVER shoplifted. It's not the type of person than I am. To be treated as though I am a lower-caliber person than I am is disheartening, to say the least. No honest, paying customer should have to face such "silent" accusations - they shouldn't be made so deeply uncomfortable just because of some employee's baseless suspicions.
You may not have meant to do this, but your post implies that being treated this way is acceptable - as long as I'm not frisked or tackled at the exit (and from my experiences, it may be only a matter of time before things escalate to this).