Pioneer Credit Union Car Loans

I purchased a car on Dec 26,2005. The dealership set me up with a loan from Pioneer Credit Union. My first payment was due on January 14,2006. I sent in a check for January's payment, and I also sent extra funds for Feb,Mar, April, & May's payments. I enclosed the coupons for these months so I would not confuse anyone on where the extra money should go. The next month, I sent in a check for June, July, Aug. Again with the coupons for these months to avoid confusion. I then sent in another check for the months of Sep, Oct, & Nov. On July 27th, I recieve a letter stating that my account has been sent to collections because I am delinquent in my payments. I called the Credit Union right away, only to be transferred to the collections Lady, who is a complete moron, and will not listen to anything I am telling her. I call back & talk to the branch manager, and let her know that I specificaly put a note in each payment to let the teller know where the funds should go. (I found out they would make just one payment on my loan & put the rest to additional principle). The manager then told me that they only allow you to pay your loan 2 mos. in advance. I told her that they didn't even let me do that! I asked her why anyone didn't call me to tell me that I couldn't pay in advance, rather than throw out the coupons for the additional months, and misapply my payments. Of course I got the stock answer "We have millions of loans, and if we called everyone who had a problem, we would never have time for anything else" Give me a break. So now, I have a scar on my credit report, which infuriates me. I have never been late with a payment in my entire life! I tried to find some place to report this because I feel scammed & cheated. But, I am only able to find a complaint center for banks. Any ideas on what I should do? I should mention that the credit union did take the money & apply it correctly, but that doesn't help my blemished credit. I know I can file a lawsuit, which I will, but where do I report this financial institution?