You never "have" to purchase an extra-cost seat. It's always optional. You could have just skipped the seat selection step and had them assign a seat at check-in. You might have ended up in a middle seat, but hey, that's one of the ways they entice you to pay to select a seat.

You can try to get it refunded but there's no guarantee they will allow it. You can try to DM them over Twitter. That is often much faster than calling.