Barclays Credit Card Complaint

Barclays Credit card department,

APR or interest rate New York

I had made a purchase -/-/- from - using my Barclays Credit Card, they offered me a 0.00 % promotion with deferred interest for 18 months. I saw -/-/- they had charged me $610.00 in interest on -/-/-, $30.00 on -/-/- & $29.00 on -/-/-. I called Barclays and spoke to their customer service agent, he told me if I pay the outstanding balance he should be able to waive the interest fully and I did ask is it okay if I make the payment on -/-/-, and he did say it was fine. So today Being -/-/-, I called Barclays to schedule the payment and get my interest waived, they started playing games, told me its too late, and transferred me over to their security department to verify if it was really me because my name is - & its a common name for a guy from -. I even told them if I was someone else I would not be concerned about interest Barclays have charged me, I would not call to make a payment and get it waived. So now I am stuck, I had a balance of about $900.00, for which they charged me interest of almost $700.00. Please help me get it waived.