Product: Payday loan, title loan, or personal loan

After my final payment on XX/XX/XXXX, I checked my dash board on XXXX XXXX to make sure that my payment had gone through. Much to my surprise the dash board said that I had a remaining balance of {$890.00}. The following morning, XXXX XXXX, I called Avant customer service to ask why their was still a remaining balance on my account as i had fulfilled the loan agreement. They stated that the remaining balance on my loan was from unpaid interest. I asked how their was unpaid interest when I had made all the payments? The customer service rep. said that they did not have access to all of my account so I asked for a supervisor. I asked the supervisor to explain how their was still a remaining balance on my account. Throughout the entire conversation the supervisor was very vague in answering questions I had. I said several times that I had skipped a payment and that is why I was being charged interest. I told him that I did not skip a payment, I was offered a Courtesy Deferment for my XX/XX/XXXX payment. The supervisor also stated he did not have access to my full account and that any further questions would have to be submitted to the resolution department, which only contacts through email. The supervisor submitted my questions and said that I should have a responds in 14 days. The following day I called Avant back to ask a few more questions that I had. The first customer service rep. stated that I was being charged interest for skipping the payment on XX/XX/XXXX and that the interest was being charged over the last two years. I again explained that i was offered the courtesy deferment. He transferred me to another supervisor and i again stated my questions. This supervisor began to explain that because I had moved some of my payments from the scheduled payment date by a couple of days that those days gain interest as well. I asked her how many days their were in total. We counted them together and came up with 66 days. She explained that the daily interest rate X the principal balance / 365 gave me my daily interest. She used the original loan amount of {$15000.00} as the principal as. Over the phone we did the calculation of .09030137 X XXXX 365= {$3.00} of daily interest. We then multiplied that by the days I had moved my payment date. {$3.00} X 66 = {$350.00}. She quickly stated that this was only an estimated total amount as you principal changes with every payment. I said OK but if my principal goes down with every payment and we used the highest principal amount for the entire calculation how is the two amount completely different? Shouldn't the remaining balance be lower the the amount we estimated since the principal decreases over the life of the loan. The supervisor quickly shut down and would no longer answer my questions and referred me to the resolution department. I have received emails from the resolution department saying that they do not charge interest on a Courtesy deferment but they have not fully answered any of my questions. The just repeatedly refer to the daily interest rate. I have tried to work with Avant to come to a honest resolution as I feel as they are trying to take advantage of me. They have not been honest and no one can answer how they came up the XXXX. They continue to send me pre-written eamils that have no substance.