When you return an item, you lose the associated credit you originally earned by buying the item in the first place.
Sound a bit confusing? Let's take an example: let's say you had previously spent $50 towards a “1 points per $1 spent” promotion, and you decide to buy a $20 item, which bumps you up to $70, or 70 points. If you decide to return that $20 item, your progress would go back down to $50, or 50 points – it's just like you hadn't bought the item in the first place.
To view your current balance, click here.