What's the recommended process when using these (or any car rental) certificate?
IIRC, the best plan is to give a credit card when picking up, and then presenting these certificates when returning the car, that way the renter is covered by the credit card should anything happen during the rental term?
If so, what happens if I rent a Premium vehicle, and then am offered a free upgrade to a higher category (Luxury, SUV, etc...)? In that case, would I still be able to use the vouchers at the end of the trip because my initial reservation was for a Premium, or would the upgraded category control?