I have not read all of this so apologies if my comments have already been made.
Yes, it's rude, yes it's bad manners. The question of who is paying does not alter that, Elvis. As you say, the checkout operator is paid to serve customers. But they are not paid to be on the receiving end of abject rudeness. Furthermore, conducting a checkout transaction is a two-way process with questions and answers being necessary from both sides. This cannot properly be achieved when one party is on the phone so mistakes are likely. But no doubt they will be the fault of the shop staff as well.
The unctuous Miss Clarke fails to see the irony of her stance when she said "I could not believe how rude [the checkout operator] was". Also, as is the case with many phone calls, it seems hers was totally unnecessary anyway. Apparently she had phoned her brother (who was waiting in the car park) to tell him “she was on her way”. Had she paid attention to her transactions she may well have been out of the store a little sooner and she could have told her brother in person that she had arrived.
The bad mannered Miss Clarke has apparently threatened to take her custom to Waitrose in future. A good result for Sainsbury’s staff then. And a bit of inconvenience to Miss Clarke as the nearest Waitrose to her is at Bluewater Shopping Centre some eight miles away. Still, it will be worth her brother making a sixteen mile round trip to take her shopping (especially if he goes on a Saturday morning when it can take half an hour to find a parking space at Bluewater). At least she’ll be able to phone him up to let him know that she is “on her way”.