Bizarre invoicing practices

We had no issues with the hired car, but were then handed over to debt collectors nearly a year later for a supposed admin fee that Thrifty had never sent us an invoice for. When we returned to them for clarification, they were incapable of providing the invoice for the amount outstanding yet insisted that we owed them. We've been sent from pillar to post, been hassled by debt collectors threatening to take us to court for a mere R400 which we had ABSOLUTELY NO IDEA WE OWED. Yet despite phone calls and emails, no one can produce an invoice.