Since I don't have to turn in my coupon at my HLE in Manhattan until I return the car, if the car is damaged during the rental, then I can be covered by Amex car rental damage insurance since technically I haven't used the certificate yet. I'd just tell the return agent to remove the certificate PC and charge my Amex card for the rental, and then I can put in the claim.

I assume if you do the online e-cert rental, you couldn't do this and would have to purchase the CDW (unless you have your own auto insurance, which I don't).

I should probably add, this is a bit theoretical since I've never had damage to a rental car while out on a free day certificate...knock, knock.