Over on OzB the idea seems ot be that you can buy a Wish card from EG to the value of $180 and ask the attendant to split the payments into 3 x $60 to trigger the $15 back. If you can find the cards in store this approach seems like sound logic to me.
Hmm, yeah, just a few tiny little things about this hypothetical process that make it virtually undoable in the real world. Or at the very least, extremely inconvenient to the point of not worth the $20 rebate.
1. $180 is a LOT of fuel. You will need a car with an at least 100 litre fuel tank and for that tank to be bone dry empty and then have a couple of jerry cans as well, to buy $180 litres of fuel in one visit. If requiring multiple visits then this negates the convenience factor massively, so multiple visits is disregarded for that reason.
Now, fortunately or not, I have just such a car. A V8 BMW 740iL which is fitted with the same fuel tank as for a V12 750iL which is 100 litres capacity. I also have 3 x BMW jerry cans with 9 litre capacity each, so I'm all set, but this brings me to point two…
2. Buying fuel on the cheapest day of the week (which is obviously what everyone who pays for their own fuel does) means packed service stations, long queues, frustrated and deliberately overworked staff in hopelessly understaffed stores, deliberately so in order to slow down the purchase of fuel to limit profit losses. Paying with gift cards (which I have done many times) in this environment is not welcome and typically comes accompanied with eye-rolls and deliberately obvious exhaling - all while there are 10 people in the queue behind you lining up for the one and only operator on duty, while the other is mopping the toilets or restacking the cigarette cupboard under the counter with a 'next operator please' sign on their cashier work station.
Let me be direct here and say that asking the cashier to split a single purchase into three on a gift card, in this situation as described, would put you at serious risk of being beaten to death with blunt instruments right there in the shop. It is up the individual to decide whether this is worth $20.