Holding deposit refund

I have received very poor service from Thrifty Rental Cars. We rented a car from 16-25 December 2021, which we picked up at George Airport. I had to pay a R6000 holding deposit on my credit card and was told that the refund would take 7-14 working days, after we returned the car. We had to return the car by 8:00 on the 25th December. Our flight back to Jhb was scheduled for 9:15 that morning. Due to Covid regulations, we were at the airport by 7:00 already, and I had to put the car keys in the dropbox by the security gates, as all the rental companies were closed that day. On the 12 January 2022, I enquired about my refund. I suspected that a fuel charge would have been deducted from the R6000. However, I was only refunded R4861. When I asked why almost R1200 was deducted (there is no way that the fuel charge could have been this much), I was informed that the car was returned late to George Airport (15:30) and that I was charged for another day!! When we actually returned the car before the agreed time, AND we already were back in Pretoria by 15:30 and celebrating Christmas with my family by then!! After investigating this, looking at security footage, etc. they told me that I was erroneously billed for an extra day and that they will refund me the R539 that was incorrectly deducted from the holding deposit. It is now almost 4 weeks, and I have not received the refund yet. I’ve sent numerous emails, but I’m not receiving any communication from them. Terrible service!!