I think that the guy was probably embarrased by his mistake which was obviously his calculations. Rather then the the golden rule of customer satisfaction, he gave you an argument.
I think you were right to bring it to his attention. I not only find this a problem in quilt shops etc, but grocery stores is another. They have a shelf price, but, it is not rung up at the shelf price. You have to be very careful these days.
I was in sales for awhile, and taught on those old bang cash registers and you had to know how to add and subtract change, and the one main thing I learned, is 'The customer is always right"
I like the others would keep the reciept and all the material in the bag and go back and request to see the manager, and explain to him/her. If you do not get satisfaction, I would be wary of shopping with them again.