I agree that since they were giving the panties away, and since s and xs are popular sizes, and since you weren't the only person who got the coupon...they ran out.

Instead of freaking out, it might have been better to suggest that they extend the offer until they restock, or maybe ship you a pair in your size. But your little temper tantrum isn't going to get you your free panties, and that's what you want, isn't it?