Payflex missing payment

I bought 2 headboards using payflex to pay in 3 months starting from January. They correctly deducted the first installment in January .The following month in February I had enough funds on my bank account for them to deduct but they didn't. Instead I received a message that I had no funds for them to deduct and they provided me with their banking details. I then manually paid because I did have their money, worse they changed me interest which I was not supposed to be charged. My Payflex account was then updated with the payment. The following month again they did not deduct but instead added interested. I did not question the reason but used the same details that I used in February to pay. I have since been receiving demands to pay from a collections company called Collier and Mallam collections from both Vuyolwethu Mbambisa and Lindle Langford who is the collections Supervisor and I emailed them my proof of payment. They both promised to investigate with Payflex if my payment was received but instead of feedback they are harassing me with letters of demand. Payflex does not have clear contact details for me to speak to them directly. Collections company on the other hand is ignoring the fact that I paid in March already but demanding a payment and I will not pay twice. Please intervene as I can not be harrassed nor blacklisted when I know for a fact that I settled my account in March already.