@BirdInFlight: As someone who has that job... it may feel super rude, but there can be situations where you have a ton of pressure on you to work quickly. I've done the sort of thing you're describing, myself.
I've got a half-dozen customers in line, all the checkstands are open, all the checkers are stressed out, all the customers are getting impatient... and something doesn't ring up. Well, we can't just stand around and guess. Not with all these people we've got to get out the door. Getting paid by the hour doesn't mean we can slack off.
"AJ, can you go get me a price on this?"
So I'm off, past a crowd of customers and looking for that exact item among dozens of similar ones... if someone's in the way they get an "excuse me" (which you should have gotten, I agree), but I'm not necessarily waiting long for an acknowledgement... some people become totally oblivious in grocery stores. I've got to get that price, rush back up to the front and resume the rest of my duties ASAP. If I wait too long, I've got annoyed supervisors and annoyed customers, and I've allowed time for the line to get bigger so now it's immediately harder for me, too.
Just a bit of perspective there. I think that clerk could have been nicer, but there's a reason for that behavior.
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Yes, I have autism. No, it isn't "part of me". Yes, I hate my autism. No, I don't hate myself.