That's just great, i'm out $18.99 too, I wonder can't you just register a new domain name with the credit through the auction? They let you backorder a domain, why not type in a domain that isn't registered, any luck there, at least it spend the money...
You can reuse your backorder as many times as you want till it actually captures a name. You cannot do this through the normal check box to back order when you search domains though. You have to log in to your account and go to monitoring and backordering, and you'll have a credit in there to reuse. Why don't you find a name drop list and try again.
AFAIK,you're not allow to use the credit to buy other thing and the credit can only reuse to do backorder domain.