Fuel Vouchers

[BENT_8]

Ok, before anyone says it, yes they probably are a scam, but that's not what this is about.
Anyway, I only do $30 worth of shopping in Coles to get an 8c voucher and I always buy heavily reduced items that I need.

So every week for like ever, my wife and I have been meeting up at a Shell servo once a week to fill our cars.
She puts about 35 litres in her VRX and I throw 50l of gas in my VT.
Always paid both together and used the one docket to claim a discount.

Anyway, this evening we roll into the local Shell servo, fill both cars, go to pay and get told we cant use one docket to fill two cars as its one customer per docket.
Crap, I told her, I am one customer paying for 2 lots of fuel.
She then said its one docket per car, to which I asked her to show me where it says that on the docket.
She scans the docket for a few seconds and then tells me she will do it this once.
I thanked her and said I would be contacting Shell direct about it.

Has anyone else had this issue, the docket only says up to 150l, it says nothing about one car.
Hell, if I rolled in with a ute with 7 jerry cans in the tray how can they stop me or determine what car/s its going in to.