As of today, you can now charge a transaction fee for customers using credit cards instead of cash or debit cards (pin-based) in 40 states. I honestly would not advise doing this - better to build the cost of credit card processing into your target margins than tack on additional fee's that may upset some customers.
Here is a link with more information as well as which states allow you to add a credit card transaction fee: http://www.ibtimes.com/new-credit-card-transaction-fees-list-states-charging-4-surcharge-checkout-fee-1042384
Are you in the US and in need of no cost retail point of sale advice? I'm happy to help and can be reached on my cell at 208-340-5632 (mountain time) or via email at firstname.lastname@example.org.