Paper or plastic?

Nice as it is that supermarkets (grocery stores) have these little people at the end of each aisle who neatly put all your shopping in bags for you, well, in fact, it's not nice at all. For a start, they always use far too many bags - as if you might injure yourself carrying a bag with more than one pint of milk in it.
But what really annoys me is that when I go to the supermarket on my bike, and I want to put all my shopping in my rucksack, it is impossible to get these people to leave me alone. For a start, often they don't understand when I say "I'm just going to put it in here" and gesture to my bag. Then if they do, they want to put the stuff in themselves. They can't pack properly, so I always end up having to take the stuff out again to fit everything in, at the same time as trying to pay the cashier. And then sometimes they seem to expect a tip for the inconvenience. Leave me alone!