Ampath Poor behaviour

Went for blood tests in January and account for R2365.30 was submitted to medical fund of which they paid R1907.20 . I then requested an invoice to settle the difference. To my surprise the amount was then inflated by almost a R1000. I phoned the call center numerous times as well as e-mail follow-ups. Only explanation is that they only later realized that they wrongly charged discounted rates. I perceive this behaviour to be ********* and tried to resolve For last 3months) with via e-mail and call center but to no avail. I insisted on contact detail for a supervisor/manager which was refused. Invoice number is L ********** . Poor and in my opinion un-ethical behavioud from Ampath.