Degrading Services

We have used Bidvest Car Rental several times, each booking we made online they gave us 200km per day. When I tried to make a booking Monday online, their system was down, so I had to call to make a booking, meaning no paper trail with all the specs and rates, they did not tell me that there are different daily km's that can be chosen, so when we received our invoice today, they charged us for 159km and told us our daily rate was only 100km and that we could have chosen a different rate, but this was not discussed before hand.