Medicross Springs Accounting Department needs a wakeup call

Firstly I have to say The doctor who helped me was first Class, no complaints there. I do how ever feel that the accounts department need to wake up. Im a cash Customer so I have to pay for service before i leave the premises, wish is awesome. I had an appointment in Feb, the Doctor sorted me out in a Gify I paid bought my script and left no harm no foal...... 6 months later I receive a letter of demand and a call from some collector agency for an a mount outstanding, how do you get an amount outstanding on a cash account? I will pay It never the less, but I think its absurd. But I wont Go there again, just now they phone me a year later and tell me I owe them my car.....