Few consumers know, or care, what the difference is between signing their name and using their PIN number when they make a purchase using their debit card. Ah, but merchants do. That simple choice you make can add up to hundreds of millions, if not billions, of dollars in annual fees for merchants. If you sign (and 61% of us do), it can cost a merchant twice as much as using a PIN (which is less vulnerable to fraud). Costco won’t allow their customers to sign for transactions because of the higher fee. Visa and MasterCard dominate the market and they set the fees, which banks collect. Those fees (the ones mentioned above and others including something called an “interchange fee”) have some merchants outraged. So outraged that some have banned together to file the largest antitrust class-action lawsuit in US history.