debit account without consent again

i booked a car online on the 6th July and paid with my card online and amount was taken from my account then i cancelled the next day as i got a better deal with Hertz, so Tammy emailed me to say i will be refunded the amount with 2-4 working day but was charged R300 just so i cancelled which is ridiculous cause i cancelled 5 days before then still yesterday morning they debited my account again and know i have bank charges also cause my account goes into a minus now, its so unprofessional when a so called big company cant even get it right the first time.