I went to the United Airlines website and looked up the terms and conditions for the two day free car rental offer for Hertz. The terms and conditions stated, that when redeeming your United Airlines rewards:
The following charges are not included as part of the Award and must be paid for by the renter:
taxes, drop off fees, extra days, applicable charges for excess mileage, any vehicle licensing fees, airport related fees, optional refueling service, optional Collision Damage Waiver (CDW) or Loss/Damage Waiver (LDW), optional Personal Accident Insurance (PAI), Personal Effects Coverage (PEC), Liability Insurance Supplement (LIS) and any additional charges which may apply for drivers under the age of 25.