Supposed interested

I opened an account in june and I have been paying every month my acc is not in arrears but they keep deducting money from my available store credit.I queried about it the first time they said its insurance of which I did not take and they cancelled it,according to me thats ********** activity..the second time they said they will reimburse me the money but they did not now they saying its interest so there is definitely something doggie here and the different stories everytime I querie aboit it.no one is giving me a straight answer they just keep taking the money every month.im highly disappointed in a huge company like Woolworths.