They done my tax my invoice show I will get R400

They done my tax my invoice show I will get R400. Back. I pay they ask R50. Every month if not in time. The lady Marie send me a mail tell me I did not issue quick enough the tax. That is not true. Then I get a mail from her Sars ask 6000.00. I call she was suppose to sort it out. They never submit my tax and says go ahead to ask there money. It's looks the owner of the business give some of the clients to this lady. I want answers I will not pay that money. I pay them R750.00 for doing my tax. The past 3 yrs I always have issues with my tax with this company.