The first thing you have to do is stay cool, calm and if appropriate, smile. Then you just LISTEN to them without interupting them.
Then you acknowledge their points, "Yes, I understand how frustrating that is." or "I don't blame you for being upset." "I am sorry you feel that way."
Then you offer solutions, and use the words solution or solve. "We can solve this by..."
Use their name (if you see it on the card)
Here are some phrases that might work...
"Mr. Smith, I understand it's frustrating for you. I wish I could complete this transaction with this card but it is not working and I have no way of making it work." Then offer a solution. "We can complete the transaction if you have another card or another method of payment." Then see if you can get them to agree or offer input, like, "Do you have another method of payment with you or how would you like to solve this?"
Wrong coupon - "Oops, it looks like we have the wrong coupon. There is no item that matches this coupon among your purchases." If they insist, you can say, "I wish I could use any coupon for any item, but unfortunately I can only enter the coupon if the item is rung up. We do have this item in aisle 4, would you like to get it before I complete your transaction?"
Price on the shelf, "I understand that you feel this price is too high for this item. I wish I had the authority to change the prices, but I don't! Do you still with to purchase it?
When you ask a question, you can tell them why you are asking. "Mr. Smith, did you find this in the sale bin? I am asking because all the items in our sale bin have an orange tag on it. It appears someone put this in the sale bin when it should be in aisle 2. I apologize for the confusion. Do you still wish to purchase it?"
Most people do respond positively when you LISTEN, display understanding and then offer a solution. There are the 2% of customers who will never be happy or satisfied no matter what you do or say. If they continue to rant you can always say, "I am sorry that I cannot resolve this for you, would you like to speak with our manager?"
Good luck!