Update: looks like it's only good for new customers.
Just got this. Not sure if you need to be a new customer or existing customers can take advantage.
Must request ride by 7/31/2018
1. Follow the steps below to get up to $30 off your first Uber ride in the U.S.
2. Tap the Payment menu, add your eligible American Expresss® Card as the Payment Method, then tap Add Promo/Gift Code.
3. Enter promo code UBERAMEX and request a ride by 07/31/2018.
Terms and Conditions
1. Promo code must be applied to your Uber account prior to taking first Uber ride. Discount only applicable to fares paid for with an American Express Card and will only apply to new users who use their account to pay for their first Uber ride. If you have signed up for Uber but have never used your account to pay for a ride, you may apply the promo code to your account. Up to $30 will be discounted by Uber at the point of sale. If the cost of the ride is less than $30, you will only receive a discount up to the amount of the ride. You will not receive the difference between the cost of the ride and the $30 maximum discount amount. The $30 discount cannot be applied to any tips that you may give your driver through the Uber app. Fulfillment of this offer is the sole responsibility of Uber. The discounted fare will be displayed on your post-ride email receipt. Prepaid and Gift Cards not eligible. Not valid with any other Uber-issued offers. Limit one discount per person. This offer is valid on all vehicle options, excluding uberT. Offer expires on July 31, 2018. We reserve the right to modify or revoke this offer at any time for any reason in our sole discretion. Available in U.S. only.