How to ruin a car rental company

I had asked for a month rental and when I returned the car (after checking with the agent in Graaff Reinet on the return date) , a week later was told I owed for an extra day late fee ! Then two weeks later was told by sms that two days were owed ! Apparently the agent thought 28 days was a month. I contacted EUROPCAR for an explanation and tried to sort it out but they sent me a lawyer letter of demand instead. Terrible company. Terrible service. Never again !