The domain industry is corrupt and backwards. I just had a domain locked for 60 days while trying to transfer it out of another registrar. They only have the right to do that if the ownership has changed, but the ownership obviously did not change, since it has been in my account with my billing information for the entire time.
Reach out to them through every support network. Twitter being the main target. Go through support agent after support agent looking for the difference in money back. Eventually it'll be easier for them to just pay you rather than losing customers from you making this public.