Refuse to meet me to discuss invoice

Helderwater installed a full home water purification system that costed me around R16 000 about 3 years ago. I had all sorts of issues but Thijs did come out to attend to it. I became unhappy as this was not normal and recently the outer bowl or filter cover cracked and they did replace it but charged me R960 for this. After numerous discussions on Whats App Thijs refuses to meet up with me to discuss the real cause of this but instead demand payment. He first said its my water pressure valve but fail to test it or to take my explanation that all my other plumbing equipment still works fine which tells me it cannot be the reason. Thijs further refuses to supply my with contact details of the parts so that I can follow up. Instead he insults me with sarcasm. I have all the WA record to proof. All I want is a meeting with Thijs to discuss this face to face to be able to find a solution.