Invoice for R24000!

Our church would regularly use Avis to hire cars for our speakers. We would pay via credit card and are always happy with the service. It would usually not be longer than a weekend maybe a week or 2 at most. Today our finance lady received a credit notice from a credit agency that she is owes R24000! Without receiving any invoices or any letters just straight to the credit agency. They saying she is blacklists and being handed over to court! No one can tell us why, which car or how it even be overdue since we use travel agents and pay upfront. Phoning Avis has been no help either. Someone has clearly made a mistake but no one is wanting to correct it. We plan to take our business elsewhere.