Do you think it's ok to use a product before you have paid for it?

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For example you are doing your grocery shopping at a supermarket. You start feeling thirsty. Do you grab a cold drink and consume it before you have paid for it (using the empty bottle) or do you wait till you've paid for it first? I have a friend that does this and it annoys me to no end. I think mainly because i'm a shop keeper myself and it's happened to me where people come to pay for the consumed product with eftpos and for whatever reason their cards been declined and they have no way of paying for it otherwise.