Booking was great, Alamo were awful

Alamo were absolutely awful. We arrived and went to sort the car, we were asked to choice the car, I asked if we just picked any, to which the man said yeah. We just choose the one he had up, it was the size we needed. He said I needed a car seat which I had to pay for. He went over the cost of the seat and that was it. A day later I checked my online banking to see £1200 had gone out of my account. When I looked on the paperwork, we had been charged £1200 for the car. I couldn't understand why as I had paid for the car. When I looked into it more, we had been charged an upgrade charge and taxes. The car we had was an upgrade. He never at any point said its was upgrade or chargeable. We went back and they changed the car for one we had paid for. The lady we spoke to said she was sorry, but didn't seem surprised. I think it was awful that we could be offered a car and not told you will pay £1200 more. What A con. We also didn't need the car seat as all the children were old enough, that did get removed from the costs. I'm so disappointed that they could do what they did. If I hadn't have gone back we would have been charged all the money.