Discover currently allows you to redeem $20 cashback for $40 Shell gift cards. Limited quantity available, first come first serve. Doesn't appear to be capped per account.
I got 18 cards, great deal!
1. Log in to your account http://www.discovercard.com/
2. Click on Cashback Bonus Tab.
3. On The Right Side, Where It Says Get It, Click "See Options".
4. On The Top, Click "Quick Find"
5. Then Under "Find By Partner, Scroll Down To See Shell $20 for $40!!