coricraft and interest charges

I bought something from Coricraft Mall of Africa on a 6 month interest free account which they confirmed to me in an email. however when i received my statement they had put me on a revolving facility where they charge 17,75% interest per annum. apparently they do it on purpose because this is not the first time they have done it. this is despite the email they send me confirming that i was on a 6 month interest free account. this is fleecing people of their money and it is bad business practice