Poor service and *****

Bad service!not even the word to use.. I dnt normally shop at there stopped to buy 70 rand mtn airtime. At the pos I asked for 70rand the cashier said the airtime dnt normally does It work,so she advised me to take two 30rands worth of airtime and a 10rand,I was fine with that. I paid and walked out. When I got home only to add the 30rand and it did not load. I called mtn and the sale adviser said that the airtime was used. I called the store,Denver Perumal(s.m)said the only way he can help me is if i bring the slip back. I just unhappy with the service pluse I must drive back to the store to get a refund.How can such a big store sell AIRTIME THAT BEEN USED BY OTHER CUSTOMERS! VERY POOR SERIVCE I WILL NEVER SHOP AGAIN IN ANY OF THE STORE'S.