I saw an argument that the "necessary and proper" clause in Article I, Section 8 lays out the authority quite nicely:
Congress has the power "to make all Laws which shall be necessary and proper for carrying into Execution the foregoing Powers, and all other Powers vested by this Constitution in the Government of the United States, or any Department or Officer thereof".
There's been a big price jump in their add-ons. Deluxe is $7.99 and audit defense is $19.99 (used to be included in deluxe, which was even a bit less)