AmeriMark i sent payments and then all all of a suden they arent getting the payments and they said i am late monroe, Wisconsin

I was sending in my monthly payments. then i got a notice saying is all i owed was 60.00 so i sent it , then i get a letter from them saying i was behind, so i called them, they said i was behind i said no i had sent all my payments in. and i have piad it off but they keep saying i owe them more money and more money i never ends. i told them they have someone ripping off peoples money but oh no it is just me , i will not send these people any more money, someone there is stilling money and they arenot doing anything about it accept rip off the customer.