As far as showing you that the difference between your tax refund and the taxes owed that is your Net, disregard. That is only an arithmetic and is not based in reality. You will receive the full amount of the tax refund and you will still owe the taxes.
If you have selected to pay the taxes owed by mailing the tax payment with the voucher provided, the program should not be asking you to pay those taxes owed with a credit card.
Or is the program asking you to pay the TurboTax account fees with a credit card?