Dynadot state in the terms that you cannot have a refund. Plus I don`t believe you entered the wrong amount. Dynadot ask you a few times to confirm the actual amount.
I have got the same problem. I need a refund of some cash as well. We made a mistake. I made a prepay page but entered the wrong number. So now I got 200$ too much in my account.
I would like to get it back. I will invest more money later for domains but right now I need it.