Standing and repetitive work
It isn't standing that does the damage, it's not moving. Stand still and your calf muscles stop pumping blood back up — that's where the leg and lower-back pain comes from. Eight hours walking a shop floor is easier on the body than eight hours planted at a till. So the first question isn't what to stand on, it's whether the job has to be done from one spot.
This is one hazard. Your register is the set that applies to you — under 20 questions, about five minutes, free.