Product: Credit card or prepaid card

Dear Sir or Madam, I bought a diamond ring on XX/XX/XXXX at the XXXX XXXX XXXX XXXX and was set up on a 12 month promotional finance program through Comenity Bank for 1year. The sales person at XXXX assured me that 12 installments according to Comenity`s payment plan was all I needed to pay off the balance with no interest. He did not give me any paper contract to sign. I got the card for the account about a week later and was directed to Comenity-XXXX  web page with a payment plan set up for me. Their web page displayed the whole payment plan suggested by Comentity stating that the first bill was due on XX/XX/XXXX and the last bill was due on XX/XX/XXXX. There were 12 total payments displayed. The first bill was due on XX/XX/XXXX. However, at Comenity- XXXX XXXX XXXX XXXX  they trigger the promotional balance to expire on the actual day the year ends and NOT on the day of the last scheduled payment. After the balance was paid off on XX/XX/XXXX they charged me {$880.00} in finance charges. They set you up on a payment plan that will not fulfill the requirements of the plan. So then you get hit with a huge bill as soon as you pay off the balance. They could have easily helped me avoid this problem by having my first bill be due on XX/XX/XXXX instead of XXXX, or they could have divided my payments by 11. They could have used their text message informational system and/or display the actual expiration date of the promotional finance program on the web page where the payment plan is. It is obvious a border line illegal, a scam like practice that they use to benefit their profits. This is unfair and unacceptable. This caused me get charged the interest on the card even though I had an automatic payment coming and was religiously following the plan. Their claim is that they informed me by putting some text in a lightly brown ( they call it red ) box on my paperless statement. This isn't good enough because the entire point of the automatic payments is I don't need to micromanage this account. For this to have ever worked I would have had to change the default billing date by knowing their system was flawed. Besides their e-statements designed in the way that they display a minimum due, a due date and a current balance carefully avoiding any information about accrued interest and actual expiration date of promotion. Once a customer sees an e-statement he/she is guided by the remaining balance, then clicks on the make a payment and directed straight to the page with the payment plan to actually make a payment. It is another trick of their deceptive practices. On top of that, they did not give me any paper contract to sign in the store, nor am I able to locate my contract on their web page. I am a full-time mother of XXXX young kids and I do not expect a stab in the back here in the USA where we value honest and responsible businesses that we trust. 
Thank you. 
P.S. Attached you will see their web page as it looked when I was making payments. The next screen shot displays the same page with " the interest charges ''. It was updated just recently. It clearly shows that they charged me " late '' fees before the last default automatic payment scheduled by Comenity was due.