from the sign-up page: "5% Cashback Bonus on purchases of gasoline made at stand alone gasoline stations"
What do you suppose "stand alone gasoline stations" are? Does Sam's gas count? What would be a nonstand alone gas station?
Cap is $400 in purchases during the period.
"Discover Platinum Gas, Home Improvement, Restaurant and Education Cardmembers are not eligible to participate in this promotion."
Thanks OP! Just signed up.
edit: called Discover and the CSR said that Sam's, Walmart, Kroger (I don't have Kroger stores so am not familiar with their gas station setup) and anything similar DOESN'T count. Exxon, BP, Raceway, etc DO count. Hope that is of some help.