Slightly off topic.....we have a similar thing in Australia where a great deal of the fuel chains are linked to supermarket rewards, i.e. if you spend over a certain amount at the supermarket, you get a voucher that entitles you to a fuel discount, typically 4c per litre filled.

This lead to service stations usually displaying the discounted unleaded price rather than the standard one. Some display both prices, some don't; in all cases, you'd be lucky to read the words "discounted unleaded" from the road as you approach the station. I know one chain makes the difference very clear.

The net result is that for those not in the know, they may pull up for unleaded petrol without knowing that you need the voucher to get that price, else it's another 4c per litre more expensive.