Europcar holding deposits as long as possible to increase profits

Hi book a Europcar through Booking.Com., paying the full amount up front and extended insurance. When Arriving at Europcar Lanseria, I was informed that (1) the extended insurance is not recognised by Europcar and (2) I had to pay an additional deposit of R12000 ("refundable within 3-5 working days). We are now on the 6th working day and ten days after I returned the car, and the deposit has not been returned ( despite numerous follow ups). Is this Europcar's business model... holding deposits as long as possible to increase profits, or is it just an inefficient process, to the detriment of customers?