I'll pay using whatever I have available (credit, debit, cash). As for discounts, get the GasBuddy app is you have a smart phone - it will use your location to show you the cheapest fuel near you.
I've learned something else as well - if I have the trailer hooked up, I go to the truck pumps at the truck stops. Pros - easier to get in/out, and the pumps run 10 times faster. Cons - diesel fuel all over the ground; wipe your feet before getting back in.
2016 Wildcat 29RLX
2011 GMC Sierra 2500HD Duramax CC
Banks tuner, airbox and filter
Rancho RS9000XL's and Firestone airbags
Pullrite Superglide 18k hitch