Interest charged due to incorrect banking details

Opened my 6 month interest-free account no hassles. No banking details issued. Used banking details printed at the bottom of the invoice assuming this is correct. Was called about non-payment and proof was mailed through. From month 2, correct banking details used. Was called again about non-payment, interest has been charged, have mailed all proof of payment about 3 times, have gone into the store, keep being told someone will call me back which never happens, have one month left of payment and there is still interest incorrectly charged. There are also charges for account protection which i was never informed of when opening account. Want to settle and close account without all interest and charges asap