You are now twisting the scenario to fit your agenda.
Presumably, the person described in the OP approached the counter and asked for the item that had been reserved online, they were charged and given an item. Acording to the law, they have just paid for the item that they were given. Whether they realised it was the incorrect item or not is wholely irrelevant, as they have been presented with an item and been told that it is £XXX to purchase it, which they did do.
I say presumably, but this is how online reservations typically work.
Presumably, the person described in the OP approached the counter and asked for the item that had been reserved online, they were charged and given an item. Acording to the law, they have just paid for the item that they were given. Whether they realised it was the incorrect item or not is wholely irrelevant, as they have been presented with an item and been told that it is £XXX to purchase it, which they did do.
I say presumably, but this is how online reservations typically work.