Why was I charged a service fee by the US Treasury when paying my taxes?

If you used a debit or credit card, according to the IRS website, the IRS uses third party payment processors for payments by debit and credit card. The third party payment processors charge fees. See the IRS webpage, Pay Your Taxes by Debit or Credit Card, for additional information.

Unlike when you use a credit card at a store, the government is prohibited by law from paying the credit card company's service fee. So you are charged for it.