Unallocated payment

I have purchased an items from a store who uses Payflex as a payment gate and the last two payments I combined as one payment however it still shows that I owe the last payment to Payflex. I have logged an enquiry that they need to look into this and have it rectified and even sent them proof of payment however they never came back to me instead they send me an email today saying that my payment is due soon. I liked using this facility but if this is the way things are going to be I will never make use of this service again.