I agree with "pain of payment"will reduce unnecessary shopping.
When we make a payment with a card, just enter the pin of sign the name. This process is going fast, we do not have chance to make the last decision that it is really necessary for me. Sometimes, we even do not care about the value of what  we will buy. we miss the process of counting cash and handing over them. I can not feel pain of giveing amount of money out, except get a receipt with lot of small letters and numbers. We just see the number of the card balance have been change, we can not immage how much money the number what has been charge means.