I'm having trouble picturing what this kind of reaction would look like. Can you explain what the cashiers did that make you think this way?
We have done a lot of reading and we believe he lacks prosody and lacks the ability to tone match. This is something other people are not consciously aware is socially important, but he routinely gets that reaction that is summed up by the phrase "I don't like your tone."
In contrast, I appear to habitually tone match without trying. This gets me read as incredibly deferential, which has a different social downside. So, one thing that does work is he and I frequently shop together, everyone knows I am his mother and people generally find me likeable. They eventually conclude "He's a nice, quiet young man." and quit having an issue with him being him.
This right here is the arrogance I am talking about. You think you know better than I do what my nearly 30 year old son needs. Now why on earth would you think that? That is incredibly contemptuous.
Additionally, letting him avoid it part of the time makes it easier for him to cope effectively the rest of the time. You can think of it as him having a limited people skills budget and not insisting he piss it away unnecessarily just because other people with a bigger budget don't find it to be a hardship to pay such things. For him, it amounts to being nickeled and dimed to death.