For federal tax filing, selecting "Charge my credit card" on the "How Do You Want to Pay Your Federal Taxes?" screen says a "convenience fee" of "2.49% of the total amount you owe" will be charged.
So if you owe $2000 for example, you are charged $49.80, which seems like a lot for your own money.
Of interest: TurboTax encourages you to use the option by saying "Earn points, miles or money back depending on your credit card." Since cash back from cash charges is usually from 1-3%, you may actually be paying more than you get back...
