The Fix payment did not reflect.

I have an account with The Fix clothing store, so what happened is, I payed last month on the 26th of July at the store but somehow my payment did not reflect on their side I dont know why. I went to the store several times and they eventually called their branch mark or something to refund me. I received my money back but now there are collections or late payments costs added now to my account because they say I didn't pay. I've sent their customer service my proof of payment even before I received my money back but till now they have not yet responded to my emails. They have been harassing me EVERYDAY with their calls and sms asking me when am I gonna pay, I didn't even know what to say anymore to them because saying that I payed was like a lie. Now I want to to go pay again but it's not fair for me to be charged late payments costs while I know that I payed on time, and now when I pay, how do I know that it will go through or not this time??? But before I pay again, What's gonna happen to those collections costs that were added because I made a payment on time?? It's been a month now and I have not received any response. I am really not happy about their customer service at all.