When we return an item that we have already purchased, it feels as if we are making money. This is because of mental accounting, a term coined by Richard Thaler, which describes how people think about money relatively rather than in absolute terms. According to mental accounting, we view money differently depending on its intended purpose. In this case, the money that was spent was intended for the item that was already purchased, so when the item is returned, there is a disconnect in our brain, making it feel like we are gaining something.

When we make any purchase, humans feel the pain of paying, which means we tend to experience negative emotions when making a payment. These negative emotions can impact our decision to make the purchase. To justify the purchase and reduce these negative feelings, our brains employ the denomination effect, which states that when a large amount of money is broken down into smaller amounts over a period, we perceive the purchase as less painful.

Girl math suggests that when shopping online and the item you want to purchase is $50, and shipping costs $20 this equates to $70, but if you spend $100 with free shipping then you feel like you have saved money. This happens because humans are particularly susceptible to the word "free," which is rooted in loss aversion, a phenomenon where humans feel losses twice as much as they do gains. If we miss out on the free benefit, we may feel negative about our purchase. By opting to spend more, we feel that we have gained the free benefit, creating the illusion of spending less.