Avis car rental Offered upgrade without proper disclosure during contract signing. St. Louis, Missouri

While I was picking up my car at Avis middle night Jan. 7, 2013, the agent distracted me with those extra coverages and gas policies. While I was busy declining those terms, I accidentally signed a contract with an upgrade from economy to full size since the agent didn't mention anything about the upgrade. It was tricky because the upgraded amount happened to be a littler more than the amount I paid on priceline.com. I thought the extra could be the airport fee and taxes. When I returned the car, I got charged with the full amount.  The manager said I had to pay because I had signed it. She said that there was no way to prove what happened during the pickup. It was a HIDDEN upgrade since it was not properly disclosed.