The best place to pay the IRS electronically is always https://www.irs.gov/payments/direct-pay
That way you have a receipt directly from the IRS, guaranteeing that you have indeed made the payment as intended.
And that's my opinion! I would never pay through any third party, no matter how easy they make it....
Of course, if you've already done so using TT, the TT expert has directed you to where you can find those details.
