Charging interest on a 6 months interest free account...

I've had my RCS Coricraft account since 2016, and opened a Cotton On account in July 2019. Yesterday I phoned RCS to get information on my accounts while on the phone with the consultant I've asked her am i paying interest on my accounts even though i always tell the consultants to do the purchase on 6 months 0% interest and she informed me that RCS do charge interest even though they are not supposed to I've checked all my statements and it did show that I've been paying for interest when i'm not supposed to.

I'm utterly disgusted with RCS and their untruthfully operation of business, on the Coricraft statement it doesn't even show how the purchases has been done.

RCS have to get in contact with me asap.