most supermarkets here pack your bags. I load up on the conveyor belt roughly how i want it packed. mostly it works.
we have another supermarket here called pack n save. as per the name, you pack your bags, and it's suppose to be cheaper. when I go there, I load up on the conveyor as per normal, and they are suppose to then pack it into a trolley, where you then push the trolley to a packing table where there are random boxes your can use or you use your own bags. Only after my UK practise, I can now pack as fast as they can scan, so i put the big heavy stuff on first ( as I wont bag that) then as per normal. Then I have to teach them about what i'm going to do, and they then scan it and leave it on the teeny tiny bit of desk beside their scanner, so i have to be fast. Upshot is, their back is saved, mine is saved, and I'm faster than normal through. I always get the operators saying they wished more people would do that.