If I plan on paying the federal taxes via online processor (lower fees than "charge my credit card" option), should I choose "I will mail a check" option instead?

[jumbojetlee]

Online processors include:

https://www.irs.gov/uac/pay-taxes-by-credit-or-debit-card

The turbotax option of "charge my credit card" shows a convenience fee of 2.49% of the total amount due.

[AnthonyC]

Yes, if you plan on making the payment using the IRS processors, then you should select Pay by Check option in TurboTax.  It will produce a payment voucher for you, which you can throw in the trash (or not print) after you pay online.