charging incorrect interest to customers

I took couches from the supplier, was suppose to pay 1100 a month because I wanted to be a great client I paid 3000 a month or 1500 on a bad month so I can finish them quickly, two months ago they told me I'm left with a certain amount, on the physical statement, went I went to pay they have increased it. I complained but paid cause I was not close enough to argue last month they told me I'm left with 1100 when I went to pay they said 500 is still outstanding, what kind of game is beares playing with our money? The worst part is that they promised me a discount and some side chairs when I took the sofas. I a young hard working girl who was just trying to help her grandmother. I think probably they think I have money. The service I got is sickening and disgusting for a company that big. The schemes they playing with people's money needs to be exposed