Negligent Staff / incompetant to do their job properly

I have went into the store to make payment on my account a week later i get called to say that my account is in arrears , i made contact to the accounts department which they require me to go to the store as the could not pick up the payment i have sent them proof of payment and a copy of my bank statement confirming the payment , according to them now my receipt shows a purchase instead of a payment which one of their negligent staff number made this mistake because i don't even have an item that was purchased on that specific day now they expect me to run around and fix a problem on their behalf where as they were at fault