If I'm doing an online order I compare prices using http://www.mysupermarket.co.uk/.
They have cashback deals on certain brands.
When I've filled my basket they send it to the Supermarket website, and I can then alter it (up to a certain time before delivery, as long as I remember to check out again).
I accidentally discovered that if I swapped something (e.g. for a different sized pack) after the order had been sent to the supermarket I still got the cashback sent to paypal.
Now they tell me I've won a free packet of tea bags. I don't use bags or that brand. Not sure if the kids do. Would it be cheating to add them to my order, then cancel? Or just playing the same game that they try to manipulate us with?