Fedex sending *****ulent invoices

For almost 2 years now we have been receiving invoices that are no for us. We have communicated this with Fedex and they have done nothing to resolve the issue. They do however continue to send us letters warning us about our overdue account and even referred us to their debt collection company, even though none of the amounts owing are ours. We have provided ample proof that the invoices are not ours.