Your cheapest option is going to be to actually use the IRS Direct Pay website. Here is a link with the exact breakdown of the service fees charged (depends on the processor you choose but overall a little less than 2%). If you go this route, just choose the option to "pay by check" in TurboTax and then make the payment online instead (make sure you indicate you are paying for 2017 Form 1040).
You can use TurboTax to process your credit card payment at the point of e-file, but the service charges are higher with this route - 2.49% (and it's not because of TurboTax - the IRS has higher fees associated with the integrated e-file and pay option). You can view those fees here -
