American Express has announced a nationwide promotion that gives Delta SkyMiles Credit cardmembers double miles on all purchases made on their card through Aug. 31.

To take advantage of the promotion, cardmembers must enroll their card online at http://www.americanexpress.com/deltaenroll by Aug. 31. Cardmembers who do not have Internet access may call 1-800-615-0403.

Normally, the card earns two miles for every eligible dollar charged for purchases at stand-alone supermarkets, drug stores, gas stations, home improvement and hardware stores, the U.S. Postal Service, Delta purchases and for payment of wireless phone bills, and one mile for every dollar charged for other purchases.

Before you decide to clear out Tiffany’s, though, keep in mind that bonus miles for this offer are limited to 10,000 miles.