Many thanks to flyinghigh77 for posting this on Flyertalk
AMEX Travel Germany is offering a promo code, jetztamex, that discounts €20 off a hotel or car reservation, with no minimum price or stay requirements. Although we’ve seen plenty of better discounts on hotels, this sort of discount can be extremely lucrative for car rentals. If you booked a car for single-day reservations consecutively (say, for a week), using this code, you could save a bundle.
Navigate to AMEX Travel Germany to search for cars/hotels. If you don’t read German, you should use a browser like Google Chrome that can automatically translate the text for you. Once you click through the payment page, you can enter the promo code by clicking on “Geben Sie den Promotion Code an” or “Enter promotional code.” Once applied, you’ll receive a flat €20 discount.