Customs department do not know what they are doing

It has been a week since my shipment landed in Johannesburg but has been held up due to them not being able to calculate the import duties fee because apparently the invoice was unclear on what is inside the shipment. Yesterday I finally got the invoice with the correct descriptions, I sent it this morning. I have been hoping to get my duties fee calculated and invoice sent to me today. I have been calling all afternoon. Now the day has ended and the email I get is one stating that they need an amended invoice with the correct amount. I'm frustrated because that is the invoice the shipper could send and there's nothing anyone can do. They wanted a clearer description, now they have it they want another amended invoice as if the one I sent is wrong. I am frustrated with them. Initially they made an overstated import duty amount of R1500 which I'm disputing, I think they want this amount to stick even though I have provided them with everything they need to make a more accurate calculation. I'm really frustrated right now I don't know what to do anymore. My shipment was scheduled to be delivered on Monday but here we are the week is gone!