Bad service

I am an Avon representative, i once paid them using a wrong ref number, sent them proof of payment and they charged me a late payment fee. Then when they allocated the money after 3 months, they are refusing to rwmove their charges saying i placed an order later and did not pay in time. At that time I paid my order however they decided to allocate my payment on that balance that I had sent them the POP which was not allocated on time... This happened last year and till today that amount is still outstanding.