There seem to be a couple of catches. First, the LDW bought through Priceline is not Hertz's, it is a separate product offered and run by Priceline, as far as Hertz is concerned, you haven't purchased their LDW and if the car is damaged they will pursue you for the charges and you will have to file a claim with Priceline (or whatever subcontractor manages the product) for reimbursement. Second, you may have trouble having the underage PC apply to a rental paid through Priceline, obviously PCs that discount the rate aren't applicable, but one for a supplemental charge may work though it's not a guarantee.