You might want to think twice before whipping out your credit card. As of Sunday, stores in most states could start charging you a "checkout fee" when you pay for something with plastic. The new fees stem from a multi-billion dollar settlement announced in July between credit card issuers and millions of merchants. Visa, MasterCard and nine major banks agreed to a $7.25 billion deal to settle charges that they were fixing credit card processing fees. As part of the settlement, credit card issuers said they would reduce these "swipe fees" -- fees paid by merchants to issuers when cards are used -- but only for eight months. In addition, the settlement also gave…