I work for a certain retailer which may or may not be mentioned in this thread. Many lols.
Not much really annoys a cashier. On for example a 8h shift they'll see hundreds of people through. Hell, everyone goes through the till. You name a characteristic short of dead and they'll be doing their shopping too. Everyone gets a smile
Even these ones:
-People trying to pass off invalid vouchers on purpose
-People pulling out a massive wad of vouchers after putting through a trolley of stuff then asking you to check if any work... let me tell you, it is NOT automatic and manually searching through 80 items whose screen names do not actually say what the product was is awkward.
-People putting a trolley through the basket tills - a trolley with a little bit in there is permitted but it then attracts increasingly full trolleys and when can you say no without looking like a ****er due to other trolleys already there.
Yes, various groups will arrange to do bag packing for tips from the customers, some are worse than others, not many are that good. They're just kids/teenagers who've only really been shopping with parents. Get about 5 mins briefing on basics of packing then told to stick to a till with a bucket. I have serious doubts on increased processing speed with volunteer bag packers.