For everyone saying it's 'nice'.... yes, it is nice that a staff member helps you lift your dog food/coke cans/kitty litter etc during your once a week shop, but the staff member doesn't just do this once per week. If they were to offer to go that extra step with every customer all day every day, it wouldn't be so 'nice' once they put their back out and lose money from having time off work to recover.
Don't get me wrong, I do think it's really helpful of them to offer, and depending on the store I would expect help to my car (eg a whitegoods place). But for somewhere like a supermarket/Kmart I think you need to be responsible for your own shopping. If you don't want to lift something then what makes you think the girl behind the checkout wants to?