Ok we're at the point now where we have one staff member on ELEVEN scanning terminals. That means that when a customer (who's scanning, packing and paying all by themself) gets a red light because of a system bug, then that customer should expect that a staff member gets in, fixes the bug, and fixes it properly.
Sure it's an important job and it might be stressful so put a second person on the job! What have you got to lose? You've already managed to shut down a half dozen terminals because the "self serve" system is working so well.
So make it work well.