You are supposed to run the whole payment process yourself. Become an expert yourself over time how to navigate all the issues and fineries of the system. You are doing a cashiers job. But no advantage for you.
Human tills have been removed and only because of that the queues on the self checkout are shorter. I am sure for some tasks self-checkout can actually be quicker but more often than not there is no difference or it can be longer.
So now you are standing there and have been good and done their job, for no reward. And once a food price crisis comes up, will the supermarket remember your unpaid contribution to reduce their cost?
Really, don’t do other people’s job.