Bad service and no delivery on promises

I took out a contract on the Vodacom Smart Tab last year for R69pm on the 1g.I was told they had no stock & tht it will arrive a week later.When I went to collect the tab I was told tht because activation happened after the deal period I now have to pay R89pm.I told the sales lady tht I shouldnt be penalised because they didnt have any stock.She said tht she would try &rectify & asked me if I'd rather want to cancel the contract.i didn't want to cancel and told her to proceed to rectify. She also mentioned tht thr is a \back 2school"promo which I can change too.Whn I phoned her again she told me she was unable to rectify & tht I must phone the Vodacom customer care line who thn referred me back to her.After numerous back & forth between her and Vodacom Custermer Care I enquired about the back2school promo she mentioned"