Poor service delivery

During Nov'14 I app**** for a vodatab R59 pm 500mb on top-up. I explain to the sales lady that the tablet was for my son. She assured me the cost will only be R59 pm if the 500mb is finished he would not be able to download anything. During the begining of February I noticed my boy was able to use the tablet anytime. I went to the vodashop to enquire about it only to find out that my account is R1600. I request to speak to the manager and she informed me that Chanel never made it a top-up but a open line. Chanel agree to pay the data usage. She assured me that my contract is a top up and it will not happened again. The end of March'15 I receive account of R2700. I went to the shop again and the sales person infromed me there's nothing that they can do I must pay the account. I feel its not my fault. Could you plz assit me?