Do I have a case against wrongful billing?

I keep receiving a bill from a pest control company for a house inspection I received earlier this year. I paid the bill in cash to the inspector day of the inspection and received a receipt. The last 3 months, I have received a bill saying I still owe this money. My fiancé and I have called in each time and explained we paid this already and emailed them the receipt. Then we receive another bill saying we are past due. I'm afraid it is going to affect my credit score soon and the company still doesn't want to talk to us more then send us a copy of the receipt which we already have. What kind of lawyer would I need?