Is it possible to fight a wedding reception contract if you feel they are lying about the liquor consumption?

Our wedding reception agreement states that alcohol consumption would be counted at the end of the night and we would owe that amount. We do not feel the numbers they gave us are possible and we are being taken advantage of. Based on the numbers given every single person who drank at the wedding would have had to consume OVER half a fifth of liquor, plus about 6 glasses of either beer, wine or champagne. Drinks were served for 5.5 hours. This amount of liquor could kill people! They gave us an estimate that we paid upfront and are now saying we owe over $3,000 more! Do we have any defense if we believe we are being lied to and taken advantage of? If it is not possible to argue the liquor consumption is it possible to argue the 20% gratuity added (as stated in the contract)?