Double-check your personal insurance. Here in New York, for example, all personal auto-insurance policies cover loss of, or damage to, a rental car, even if the policy does not cover loss of, or damage to, one's personal vehicle.

If you only have Amex "secondary" rental-car coverage, that coverage will be primary for you if you have no other collectible insurance. (If you are enrolled in Amex's optional Premium Rental Car Protection Program, then you have primary coverage from Amex regardless of the status of your personal coverage.)

Depending on how significant the crack is, and how long your rental is, you might consider bringing the car back to a Hertz office now, and exchanging it for another vehicle. 😉