Car Rental Overcharged

Purchased a car rental through their website from Dollar Thrifty. Was required to pay a deposit on the rental. Travelstart issued a voucher for rental displaying the deposit and amount to be paid when collecting the vehicle. When collecting the vehicle, the deposit was not deducted from the rental and Dollar Thrifty charged me the full price of rental and claim they cannot see the deposit, or display it on an invoice, thus I have been charged extra.