For those who are American Express cardholders and Costco members, they are offering a $10 Costco Cash Card for paying your eligible wireless bill automatically on an American Express Card. To receive an Amex$10 Costco Cash Card Bonus, simply sign up any eligible American Express card and filling out your information by March 31, 2012.

From the website:

To be eligible for this offer, register any eligible American Express Card from now until March 31, 2012 on americanexpress.com/costcophone. You may only register one Card for this offer. Using your registered Card, set up automatic bill payment with your wireless provider. All wireless providers distributed by Costco are eligible. Participating providers as of July 5, 2011 are AT&T, T-Mobile, Verizon Wireless, Sprint and Alaska Digitel. Participating providers are subject to change. Prepaid and corporate cards, and ExpressPay transactions, are not eligible. If your Card is replaced during the promotional period, please call the customer service number on the back of your Card for assistance. Limit one Costco Cash Card per Cardmember.

Allow 8 to 10 weeks after your first wireless payment with your registered American Express Card to receive the reward of a $10 Costco Cash Card. Sign up today for an easy $10 bonus!