Smurfette

I was wondering if anybody can please tell me what my next step should be.

I sent my host the money to renew 2 of my domains back in Jan.
One of my main ones and the other was just a domain for doorway pages that did have some old sites on it that weren't being updated. I only noticed that my big one went down and they told me that one didn't get renewed, so they gave me 1 free months hosting. It wasn't until the other day I went to go to the other domain that I noticed it wasn't my stuff on the page so I did a whois on the domain and it wasn't registared to me anymore.
I have the reciept of payment for the both domains to be registared and one didn't get registared and now somwone else bought it.
I called my host and they said it was a problem on my end but I was talking to the guy in accounting on friday and he said yes that they had got the money and he went back and looked at it and don't know why they didn't get renewed and he would check into it, after the weekend of not hearing from them I called tonight and was talking to the owner and he told me it was my mistake even though this has happend twice before but caught the other times. He offered me $15 for the fuckup. But I lost a good domain that was getting a lot of SE traffic and was making me money.
Can anybody give me advice on what I should do and do I have any legal rights?

Thanks in advance
Smurfette