I got really messed over when trying to use the voucher in Maui. The counter rep took the coupon and forgot to apply it to the RSVP. So I was charged an additional fee for dropping the car off at a different location.
-after arguing with Hertz for a month eventually they credited it back to me.
To all those using vouchers, just make sure you verify like 100x that everything is applied. Also if you are using your credit card for insurance purposes, I believe the proper route is to use the voucher and if anything were to happen, have them delete the coupon from your reservation.