The Incredible Gift: "You've won an Amazon voucher!"
An advertisement pops up or a message arrives: "Congratulations! You have been selected to receive an iPhone or a 500 euro shopping voucher". They ask you to answer a short survey and then pay only the shipping costs. Unfortunately, no one gives anything for free on the internet. It's just a trick to activate expensive subscriptions on your phone that will deduct credit from you every week without you noticing. If it seems too good to be true, it's probably a scam.