Unfair marketing and dealing with customer

I went to Sterns in Circus Triangle, Mthatha to pay my Markham account. The lady who assisted me told me that it is a rule in their store that if you want to pay another store account, you must buy airtime first. My reply was 'ok' and I left to American Swiss in the same mall and they helped me. What they are doing at Sterns is against CPA, it amounts to unffair practice to customers and unfair marketing, that in order to accept your payment you must buy something, it's wrong.