I've purchased fuel at Shell servos recently, and used my Coles 4-cents-off voucher.
On the last two occasions, the attendant has offered me a 'free' drink.
I turns out that what he means by 'free' is that the cost of the drink is about equal to the discount I've received, so the final bill just comes to the undiscounted cost for the fuel.
Not bad ... it's sucked me in.
I don't really object, but it is a little bit cheeky