My son and I used the "Official Paymens" company to pay my son's outstanding IRS bill. We started at the IRS site and were steered toward Official Payments. We provided the credit card info, and made the payment to Official Payments, along with a $2 fee, for which we were sent an email from them with a confirmation number.
That was September 5, 2018. On October 9th, my son received a registered letter from IRS with a collection notice for the amount he owed plus interest. The confirmation number means nothing to IRS, and they say that my son is on the hook for the payment and interest, until it is paid.
We should have paid IRS by mail, and will never use Official Payments again. We are filing a complaint here, and with our state's attorney general, and trying to get the bank to reverse the charge. One would think that IRS would vet companies like this.