1st car rental

I've hired Ford Figo for 6 days on R429 daily rental, I've requested extension only for a day and the funds was deducted immediately. On my email I clearly stated that will return the car at 17h00. There's some funds that they are holding. I return their car on good condition when they had to refund me, my refund was short and I've inquired and they advised that when I took the car it was at 11h58 and I was supposed to return it by the same time and they only have 1hr grace time. To my surprise how can you charge someone for 4hrs R429?? Which means for a day I've rented FORD FIGO for R858..it's worth it? I feel this it's a day light *******, I will never ever hire from 1st car rental and I don't recommend anyone to consider them. I am so disqusted with their service.