Hertz - Expiring Cust Satisfaction Certificate

I have two Cust. Satisfaction certificates that expire 5/31/09. I won't be able to use them in time (kept forgetting to use them on prior rentals!)

Has anyone been successful in doing any of the following:
-send the certificates in for an extension
-send the certificates in to apply against a recent rental charged to cc
-prepay a future rental (starting after 05/31/09) using the certificates

I don't think that you can do any of those. However, you may find locations still willing to accept them. Those certificates sould be charged back to the location that issued them so it is money either way for the location you are renting from. At least that is what I was told but 90% of the lines on a Hertz P&L cannot be explained by the people who issue them.