If you walk to the supermarket you've no doubt noticed how the thin plastic bags given away for free can cut into your hands if they have more than a couple of kilos of items inside. If you have a carabiner you can carry several of these bags comfortably without the plastic handles digging into your hands.
Reddit user lathesand shares how a climbing carabiner can hold several bags, but you can also use the cheaper keychain type. Also if you carry a Swiss Army Knife many have a small package hook that can also serve to carry one or two of these bags.
Either way it beats going up five flights of stairs with six bags trying to flay the flesh from your hands.