I'm probably going to jinx myself, and I rarely purchased many items at a store anyway, but it seems to me that they load the merchandise in the order they scan it. I might have some beverages that are heavy in a case of 12, and I put that on the belt first. I think they go ahead and bag those as they occur.
Have you left something you paid for at a store recently?
That is probably my biggest concern if I am actually purchasing several plastic bags worth of merchandise. I can never tell if I have them all or not. I have caught them telling me that I have everything before and I reached around and found a bag with some of my paid merchandise in it, so they are not foolproof.