Overcharged refusing to payback

I would like someone to get in touch because all I get is generic emails without an explanation. I went for blood tests within the network, Ampath then charges me full amount saying it’s not covered. I query this with Discovery stating that I was only meant to be charged a specific portion, and that the rest is neither payable by Discovery or me, the thing is I had already paid in full, so who exactly must give me a refund of what I overpaid? I messaged both Ampath and Discovery and neither is taking responsibility. The generic response I got from Ampath is as follows: Please be advised that we will not be able to refund your credit as you have no credit due to be refunded. Ampath is the one who did not do their due diligence to check what the plan covers and what it doesn’t, so why are they treating this like it’s not their fault? ( Discovery Flexicare Plus plan)