Charged for their poor service

I returned my car to their designated car park on time but then had to wait behind a queue of other customers to have my car inspected and signed off by the inspector. There was only one inspector at the site - Olbia airport - as you can imagine a site very busy with tourists collecting/returning cars. It took just over half an hour to get the car inspected and signed for. The inspector told me everything was perfect and there would be no charges. Once I'd flown home I received an email from them with an invoice for an extra day's hire - in effect,  have been charged for their poor service and procedures. I have emailed their customer services in both Italy and the UK several times with proof that I returned the car on time but they just ignore them - they have my credit card and think they can do what they like with it. Stay clear of this sham of a company!