Buy Gas With A Card That Pays Cash Back
Discover Card that you were talking about can help you save not jut 5%, but up to 10% if you redeem your cash back balance for gift cards or shop at Discover shopping network. Theres also a BP card that saves 10% and a few more that save more than 5% on gas...
Credit Card for Person with Bad Credit?
Check out http://www.creditcardmix.com or http://www.cardfusion.com. Those guys have special credit cards for bad credit categories in their catalogs. They also have prepaid and secured credit card offers, if you are interested.