helzberg diamonds comenity bank charged interest incorrectly texas dallas

I purchased two rings a wedding band and engagement ring from Helzberg diamonds.  They were offering an 18 month no interest.  The first ring was purchased in jan of 21 the second was June of 21 the interest free was for both purchases the first purchase was 18 months the second was 6 months. 

I had an outstanding balance last month of about 160. I received a bill for 1253 today stating they are charging interest from day one because there was an existing balance.  So my bill went from under 200 to 1253 on a technicality. 

I called them and they said there is nothing they can do.  This is a robbery what they are trying to do to me, infact when i purchased t he rings i was going to pay cash for the rings and they pushed heavily on using their interest free credit card it is very obvious why they did this. 

The origional purchase on jan of 21 would have been paid off several months before the final date.  The second purchase if i am to be charge anything it should be on that purchase not the first.  This is a terrible buisness practice to do this to a good paying loyal customer to be told theres nothing they can do because i missed the grace period by a few weeks.