Synchrony Bank Credit Card Complaint

Synchrony Bank Credit card department,

Advertising and marketing Pennsylvania

I was offered a card that had - interest for the first 6 months, I was told by the store merchant that at the end of 6 months I would be charged interest. It was never explained to me that the interest would be compounded. So now I 've noticed that I was charged one lump sum of over $300.00 in interest plus on going interest at - %. I called to complain and offered to pay the entire bill and they refused to waive the lump sum interest. My issue is the lump sum interest being tacked onto my account.