Disappointed by a big car rental company

Made a booking for December for a 4 days. There was a special going, if you book for 4 days you get day free, I did not get that. Nonetheless, I paid for my rental, no invoice, nothing. I do not know how much the car was per day. I just got the total amount and deposit that I had to pay. My deposit was finally sent to my account minus the petrol amount, which was at most R100 and way less than what I had expected. I do not know what was deducted and why as I did not receive and invoice.