Paid off...but is it really? #Discoverycard

I have had ongoing problems with paying off this card. I got quoted settlement figures which were double the outstanding balance, when I asked about it, on numerous occasions I was told "Interest and other charges" . i let the installments run as normal ( instead of paying off earlier and paying exorbitant interest) and paid the last installment in April 2016. Early may I called to check if the account was going to be closed as I had paid the last installment. " No, it won't you still have to pay interest" When i asked , the consultant couldn't give me an answer. I asked for a breakdown of their workings and how they figure I still owe interest. I never received that email with a breakdown. i am tired of calling, wasting my airtime and tired of forking over money for nonsensical answers.