yabu. You are not responsible for the woman, but you are responsible for your own child, so whether or not the woman was rude doesn't matter - you teach your son to be polite to everyone, rude people included.
this means: not dicking about with a door which people need to use to access something that matters to them very much (medicine), that they might be feeling emotional about, while they are in a hurry
not treating everyone in the world like accessories to his me-me-me fun-tymes
not insulting other people by pulling them up on manners (it is never polite to talk about others' manners, no matter how bad they are, except your own children whom you are teaching)
not patronising people who are older and wiser than you (he did it by mistake, but you need to tell him that he and adults do not have a symmetrical relationship. you can tell him "it is nice to say thank you" - he can't talk to adults like that, ie, presume to instruct them about anything other than privileged information about himself eg "I am thirsty" "I don't like that")
However, having said all that, I don't think the woman was rude to ignore him in the first place. I think she was polite to ignore him dicking about being a pert little pain in the arse, rather than telling him to get out of the way
what some people do not realise is that interacting with others is, for some people, extremely demanding. Interacting with people who are demanding that you be "nice" and "fun" is a job. If a person doesn't feel like it, at the chemists of all places, they shouldn't have to do it. you can always say "please" and "thank you" but people of all kinds and ages can just be too demanding when they want more, in the vein of "how are you this fine day" [twinkle twinkle] or "cheer up it may never happen" or what your dc did. It is ok not to play. It really is.