Avram, you should not have a cart in line and be out shopping at the same time it doesnt matter if u need 1 item or 20, making out a shopping list and following it is not so hard , i do my shopping and when i feel i am done i go to the checkout there is no reason for leaving the line to get that item.
You are correct in thinking u can get that one item and be back b4 its your turn to checkout but by doing so u are saying to your fellow shopper i know i should have done my shopping b4 getting in line but the rules do not apply to me, this very attitude is a big contributor to anti-semitism