Concerned about interest charges.

The treatment I get from this company is very concerning. First of all, they debited my account before the date we agreed on and for that, I was penalized by my bank for the debit order that did not go through. After I fought to have them sort that out, they always received my payment though I paid, and charged me more interest on my loan because the money that I paid to them does not reflect on their side. I am talking about paying on the 3rd or the 28th of the month and sending POP every day till Jesus comes and still they never received these POP and therefore the payment is not allocated to my account Now my concern is, if I pay on the 3rd of the month and the 15th they are sending out statements again, and on their side it the money that I paid on the 3rd of that month does not show, does that mean they are charging me more interest? I have been calling them and emailing them about this issue and they never responded.