Unhappy client Car Trawler and Avis - no refund

I made a booking to collect an AVIS car from East London airport on 6 May 2018 and to be returned on 8 May 2018. I initially made the error to say the return date is 13 May. I did attempt to correct this online immediately and honestly thought this would be allowed, especially in line with Treating Customers Fairly principles and the Consumer Protection Act. Despite the law, this amendment was not allowed. I then attempted to make the correction once I collected the car. Once again I was told I must contact the AVIS call centre - ***, which I did this morning. I was then told that Car Trawler does not allow corrections. I must pay for the days that I do not require despite my attempts to correct. I find this absolutely unacceptable. They have taken my money and will not make a refund. Avis is saying this is a car trawler issue, and car Trawler's website will not allow a customer query to be sent. They say their system is down.