No, I wasn't being serious, Hiding. I just wanted to make the point that you shouldn't take it out on the checkout girl and take it to the powers above. Contrary to what you say further down, this most certainly does not happen every day and, when it does, the cashier is often visibly shaken. The younger ones, in particular, have to be taken off in tears, if the customer is particularly aggressive towards them and you get all sorts of complaints from pricing to not stocking items to the way the shop is laid out, none of which is the decision of someone earning £7 an hour.
Most of the staff are students and are even younger than OP's daughter and it is not very nice to witness. I am not sure if there is actually a nice way of doing it and it's pretty pointless anyway. You yourself have to do your shopping all over again instead of taking the shopping you already have, buying the wine elsewhere and writing a stiffly worded email to Head Office, who may be able to actually do something if enough people complain in writing, rather than throwing a tantrum at the till. And you'll get an apology and vouchers. I know from experience that complaints rarely get passed up to hq - even from the managers.
And, as I said, it is relatively rare for this to happen and is often the gossip of upstairs and, mostly, the customer is perceived in a very bad light for behaving in this way. We have had the odd customer banned from the premises if they are particularly abusive. They can't just behave as they please.
As for it being my job, well, yes, effectively, someone has to do it but what many customers fail to remember is that 99% go around, do their shopping, are pleasant and then go home and those are the ones that make us enjoy our jobs and wish to work hard for.
And finally, with regards to taking on more staff, naively, yes, perhaps they should but they don't. Supermarkets work on tight margins and if they can get the minimum amount in for minimum pay, then they will. That's not the fault of the customer but that's the way it is.