We human beings are social animals, so we like to make sure that we always pay our debts. For example, if you know that the person you’re meeting is making a long journey to get to you, you’re more likely to make sure that you’re on time. This is because you’re aware that they’re contributing something to the deal, so you feel the obligation to return it in kind. In the world of e-commerce, this is a trick that can be incredibly simple to use: you make your customer feel like they’re getting special treatment, you’re more likely to get them to make a purchase. By doing so you apply the reciprocity principle.