Harassment over already paid invoice

My insurance company hired a car through Avis in October 2017 while our vehicle was being repaired. We returned the car and there didn't seem to be an issue. Now we get calls daily because they've handed us over for non payment. For the passed 3 months we've requested that they send us the invoice so that we can see what it's for in order to make payment. Further to that, my insurance company has been fighting with them because we want the invoice. They initially sent us an invoice that we proved we had paid with a credit card at their office. Now they are quiet. Yet we get harassed daily because they've handed us over for this R900 odd. We will now be taking this to our attorneys as we have everything in black and white. This is harassment!!