Bad service and customer exploitation

I rented a car with rental agreement E*** and used the car for two weeks. The day i returned the car there was no one to inspect the car with me as there was no one on duty yet it was during working hours. A day later i received an invoice of R7800 which was deducted from my card. After a few days i received an invoice which they said is for damages and it amounted to R10500 of which i made an EFT transfer to them. Now i received a letter from Avis saying i owe R9400 the letter was sent to me by a lady with the name Melisa Woest. Upon this i phoned Avis each and everyday since receiving this letter because i want to know what this amount is for and how they arrived to it,i have sent more that four emails but there is no response whereas they have threatened to escalate the matter to their attorneys. All i want is clarity as to what is this money for as i cant pay for something i have no information on. This situation is frustrating me as they keep on debiting my card causing me to incur unnecessary bank charges. I am so disappointed and frustrated with their treatment. Their call center keeps on transferring my calls from pillar to post.