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.
To get them, log into your account at www.discovercard.com
, click "redemption center" at the bottom -> partnered gift cards -> home, and it is on that page.
Conditions: The Shell Gift Card can be used only for purchases of goods and services at Shell locations in the U.S., except territories. The Gift Card cannot be redeemed for cash and no change will be given, except in states that require redemption for cash. Shell Oil Products U.S. and its related companies are not a sponsor or co-sponsors of this promotion.