avis rent-a-car avis rent-a-car RIP OFF orlando, Florida

I arrived at Orlando with my family at night, following a long flight. I had pre paid for car hire having organized a dodge van or similar to meet our holiday needs. This was arranged  through British airways who allocated Avis and a prepaid voucher for my car hire . I signed a car hire agreement at the Avis desk and collected a Lincon which was very similar to what I requested and thought nothing of this. When I returned to the UK i checked my statement and was shocked to find Avis had debited my account for an additional $420.01. I made enquires through BA and Avis and was horrified to find Avis had upgraded my car hire without my knowledge or discussuion with me.However, on my Avis statement the additional charge was against milage and distance used. When I pre booked my car hire the BA site stated all car hire was unlimited milage. I feel this was rip off because at no time did a discussion take place that I was being upgraded , or discussion of charges that I  would incurr . I would like to warn people who give their card card details and are using Avis do not to sign the car hire agreement without throughly checking and reading through ,do not assume what you thought you  booked is what you will get , and beware it is not until you get home you realise you have been conned out of more money. I wanted to highlight this con as I feel sure I am not the first to be riped off by Avis.