You have two conscious choices you can make every day: be miserable, or do something about all the things that are going wrong. So you're afraid of shopping because you're afraid of interacting with the cashiers. What exactly are you afraid of? What's the worst thing that can happen, really?
As soon as you dismantle the bomb that is mind-fueled anxiety, freedom becomes much closer.
I've had some pretty upsetting experiences with cashiers. Quite often (similar to what you mentioned in another post in this thread), I receive an increased amount of scrutiny from them than other customers seem to receive. And don't insult me by telling me that it's my imagination; I have eyes, I can see that they are staring at me or giving me other (or more subtle) forms of increased scrutiny. My perceptions are not that off that I cannot accurately determine such things.
Just one example: a cashier at a pretty popular regional grocery store chain (I'd love to name the store, but I won't) literally gave me a 10-second stare-down. Shortly after it was my turn to be checked-out, and he was about to start ringing up my items - he just kind-of stopped what he was doing, and stared at me for a ridiculously excessive length of time. Things got so excruciating and awkward that I fumbled for something to say to break his gaze. It was only after I awkwardly said, "Why are you continuing to look at me?" that he kind-of broke his stare off and said some silly, "customer-service" line that escapes me right now (I probably don't recall exactly what he said because my anxiety/stress-level was so high from his rude prolonged staring).
I've had very similar things happen to me much more than they should; cashiers giving me a certain stare-down. For someone with social anxiety as severe as mine - such treatment is a big deal, and is extremely difficult to take in stride. It is torturous for people in my predicament to have experiences that reinforce my fears about interacting with people, and confirm certain ideas that there is something "odd" about me that encourages or emboldens people to stare at me.
So, certain "fears" about cashiers may seem unwarranted to you and to others whose anxiety does not cross a certain threshold of severity - or who have not been treated in a discriminatory manner by these employees. But for those of us who are struggling with certain severe symptoms - and have had the unfortunate experiences that confirm our fears - such fears are quite realistic.
A major part of the problem is that employees have been drilled to be on the lookout for possible shoplifters - and they have been trained to see certain behavior (even if such observations are based on nothing more than a "gut feeling") as "suspicious". S.A.D. sufferers who are unlucky enough to not be able to hide our symptoms well enough - or who are unlucky enough to somehow have other characteristics that are misjudged - can be the unfortunate victims of this toxic retail atmosphere.