Sliceit com - Late payment fraud

I used their credit system * time to pay for something and instead of * transaction there was * transaction. So I complained again and again and they didn't respond or solve the issue and the due date was comming I asked then again and again they didn't respond or solve and at last I even said that I won't pay any extra money, when the due date crossed they started to charge me late payment fees and then they contacted me and they resolved the transaction issue but that also only * transaction was levied off instead of * and they were asking me for the late payment I asked them to explain me why I'm entitled to this late payment they just reply with "no sir you have to pay" I have all the screenshot of their chat support with date and all.