When you pay in the FILE section, you should be able to choose to pay the TurboTax fees out of your Federal refund. That option has a 34.99 service fee (39.99 fee for California filers, where it is part of the the Premium Services bundle.)
That option requires efiling and direct deposit of a refund. If you selected to get your refund by a check in the mail, or plan to mail your Federal return, then you cannot pay your fees out of the refund.
Your Federal refund must be large enough to cover the product fees, any sales taxes (if applicable), and that 34.99 (or 39.99) service fee I mentioned above.
You must also have a US address.
See this FAQ for other requirements: