You have the choice of three different processors, and the rates go from 1.87 to 2.25%. Please keep in mind that the convenience fee is imposed by the IRS and not TurboTax. More information below:
https://www.irs.gov/uac/Pay-Taxes-by-Credit-or-Debit-Card
You can also choose direct debit, which carries no charge but the IRS will take the entire amount owed out of your bank account.