During a recent rental I had the car for a period of 2 weeks, 1 week at first, then I extended 2 times to get to the 2 week duration. Initially I ha to put down a $250 deposit, which I understand. When I went to extend it the first time, I asked specifically how much extra would it cost me to extend the rental for x number of days. They gave me a price that I was OK with. For the next extension I asked the same question. They told me that the price that I was given for the first extension was what was left from my deposit, not what the additional charge was going to be. This ended up costing more that I though it was since they did not communicate it correctly to me.
I my option the deposit should be separate from your payments, there only if they need to use it, not something they pull money from. They did not communicate the additional cost to me correctly at all.