Product: Credit card or prepaid card

I bought furniture from XXXX XXXX XXXX. The amount of my purchase was slightly below {$7000.00}. I was given a promotion, explained clearly to me by my sales person, that XXXX XXXX was offering a 1 year sales promotion.. As I approached the end of my offering period, my monthly statement indicated that my account - quote " you must pay your plan balance of {$3600.00} by the end period date. '' - end quote. 
I paid a total of {$3200.00} by that date. I actually misinterpreted the balance, because there were pending payments on my account. 
The instructions also informed me of the calculated amount of interest to be paid if the balance of {$3600.00} remained on the end date. The amount of interest was approx {$1400.00} to be paid on that {$3600.00} balance. I reduced the balance tremendously by {$3200.00}. 
I received a statement l7 days later that I owed {$1800.00}. 
I called them, and they told me that my entire balance was not paid by the end date, so they charged me the full interest amount. 
This did not hold true, and they should not have charged me for the following reasons : 1 ) If that was true, then they should have charged me interest on the full amount of {$7000.00} and not the balance of {$3600.00}. I have paid off most of the {$7000.00} leaving {$3600.00}. Again, I paid off most of the {$3600.00} leaving XXXX. They should again decrease my new plan balance to XXXX, and charge interest on only XXXX. They did increase the XXXX to {$3600.00} but they did not increase the {$3600.00} to XXXX. All these payments were made before the end date leaving a balance of XXXX. Interest was charged on {$3600.00}. 
2 ) They did a tabulation of what my interest would be on {$3600.00}. The tabulation indicated that my new plan balance of {$3600.00} would be charged interest of approx {$1400.00}, almost half. After that statement date, and before the final promotional ending date I paid {$3200.00}, so at the end date, my new plan balance should be reduced to XXXX. I should be charged interest on a balance of {$490.00}. ( The tabulation is called Details of your plans '' ) 3 ) Nowhere in there notifications did they mention that would be charging full interest unless the entire balance is paid. On the contrary they had already reduced the promo balance earlier and is refusing to reduce the promo balance any further, even though it is paid before the end promotional date. 
4 ) They did not indicate that the accrued interest date for their final calculations would start at the beginning or the middle or any time during the loan. However they picked the middle to do their calculations ( the loan started at $ XXXX ). They indicated that the promotional plan expired XX/XX/XXXX and new promotion starts on XX/XX/XXXX. They indicated that my new promotional amount on XX/XX/XXXX, is XXXX ... then it would make sense for them to match the new promotional amount ( XXXX ) to be matched with the new promotional period ( XXXX ). Everything starts all over with the remaining balance, so interest should start acruing at the start of the new period. 
5 ) The rates charged after the promotion is 30 %. The interest they charge me is {$1400.00}. This is on a balance of {$490.00}. 
6 ) Accrued interest as stated on their statement should be accrued on the pending balance and starts at the current balance date. You can not accrue on a loan that has already been paid. 

Their statements and promotional plan is misleading and dishonest. They do not tell you the truth via their sales associate as well as their information on their paperwork. 
What sense would it make for me to pay {$3400.00} to reduce a loan blance of {$3800.00}, and leave a {$400.00} balance for them to charge me {$1400.00} plus 30 % interest. Only if you are tricked or crazy, you would fall for that. Please note that I foolishly recomended 3 clients to them, one being my sister who spent more money with them. I also paid my bills on time every month until I made the final payment. 
Today I issued The balance of {$490.00} to them along with the {$6.00} interest accrued. As of today I should owe them {$0.00}.