I believe jason8612 is correct about having to physically present it at the counter (or at return), but I would say it's worth considerably less than $50 because in order to use it, they have to replace the promo code you used on your reservation with the promo code on the cert. If you used something like a "free weekend day" promo code to take a $30 day off the rental and then use the cert, they'll remove the free day first (adding $30 to your rental) before subtracting $50, making the certificate worth all of $20 in that case. Crappy, but that's how it works (and why you should always push for an actual cash refund in the event of a service failure from Hertz rather than accepting their funny money).