The system knows the weight of the grain container before the customer takes any grain. It also knows the weight of the container after the customer takes some

The system knows the weight of the grain container before the customer takes any grain. It also knows the weight of the container after the customer takes some, so it's not too hard to figure out how much was taken. There will be exceptions where this is not the case (e.g. grain accidentally spills), but it will work 99.9% of the time, and then as the article mentions, for the other times there will be an attendant to sort out any issues.