Bad service from dstv

My contract should have ended last month with DStv for price lock and I have been sent from pillar to post because when the contract started the person that captured my contract captured wrong amounts and I was not chargers the correct amount. The total is +-400. What is a shame is that for many times when I was enquiring dstv never brought it to my attention that there was a shortfall. Now that the contract has ended , they want me to pay for their mistake. Sooo shocking and very disappointing. Why am I carrying the costs of their mistake ?