Amex Platinum is Hertz CDP# 21176, though I'm pretty sure it doesn't wave the under age fee for being under 25. USAA (CDP#77694) will give lower the age to 18 and possibly be an even better rate than the Amex Platinum CDP, but you won't get the "4-hour no-charge grace period" if you don't use the Amex Platinum CDP (and you can only use 1 CDP). There's also PC# 144314 that lowers the rental age to 20 and can be stacked on with a CDP.
With Priceline, while naming your price you put in your age, and if your offer gets accepted there is no additional fees for underage.