I normally use AmEx premium rental protection (the $25/rental protection) instead of insurance provided by rental companies, but I always book through the company.

On an upcoming trip, I decided to save some money and play Priceline Roulette, and snagged a good deal on a rental through Hertz (about half what it would cost me if I booked via Hertz). I paid for it with my AmEx, however, since the charge was from Priceline and not Hertz, the $25 premium wasn't charged so I guess I can safely assume I'm not covered.

So, just wondering if there's any way to activate AmEx insurance for this rental, like do I need to call before my trip, or will it be charged when Hertz swipes my card for the hold? Or am I stuck paying for CDW or risking a claim on my personal auto insurance? Thanks in advance.