What is the amount of the convenience fee if I use a credit card to pay the Federal taxes due?

A convenience fee? I'm not really sure what your asking, sorry.

Ha! It's on the Turbotax website that paying by credit card includes a convenience fee...I found it on the next page...it's 2.49% of the amount owed.

Here is the layout of what the percentages you are charged from the IRS.
https://www.irs.gov/uac/pay-taxes-by-credit-or-debit-card