Harassment About Unclear Accounts

This is the worst company to deal with ever. No invoices was ever received regarding outstanding amounts, just constant emails about outstanding accounts. No one responded to any emails sent. Finally managed to get through to them, spoke to a lady named Tanja and explained the situation. She sent an invoice which was paid. Still emails keep coming in regarding outstanding amounts. Called them again and not only is the person assisting rude and refusing to look at the matter, but she tells me there is more outstanding wothout wanting to tell me why.