Santander Consumer USA Santander is refusing to refund a car payment. Fort Worth , Texas

Santander Consumer USA is refusing to refund a over payment. On June 10 , 2012 my car was paid in full and I accidentally send them another car payment. I have always paid my car note 30 days in advance and never late. Its a good thing I always sent my car payment Certified Mail. According to Santander they have no record of a payment being made for the month of June. I have called and emailed Santander and to this day no one has responded. Its a very simple request give me back my money !! After doing some research on line about Santander  its unbelievable how a loan company treats their customers. So here I am complaining hoping to get some resolve.