The booking you make through the AMEX site probably has a CDP number associated with it for your company , giving the rate that you pay

Should you go and modify the booking to add the Marriott CDP, that rate will be affected and price may well increase

You can compare by doing a dummy booking for the same rental , using the MArriott CDP, on the Hertz site

If the marriott rate is cheaper, perhaps you can get approval from your company to use that rate instead