Product: Credit card or prepaid card

Background/Context : In XX/XX/XXXX, I opened a credit card through online retail company, Wayfair. At the time, Wayfair partnered with Comenity Bank for their credit cards. 
In XX/XX/XXXX, even with a balance left to be paid, Comenity Bank closed my account. Later on I learned this was because Wayfair discontinued their relationship with Comenity Bank for their credit card accounts. 
Since XX/XX/XXXX, I tried to pay down my balance more aggressively by paying more than the minimum required. 
In my XX/XX/XXXX statement, the new balance was {$1700.00}. Payment was due on XX/XX/XXXX. 
o On XX/XX/XXXX, I scheduled a payment for {$200.00}, leaving a balance of {$1500.00} before the statement period due date. 
o On XX/XX/XXXX ( the date due ; and within the statement period ), using Comenitys own electronic payment scheduling system, I scheduled another payment of {$1500.00}. I wanted to be done with this Comenity account once and for all, so I decided to forego life essentials and other payments for XXXX in order to make that very substantial {$1500.00} payment ; but I did so believing this {$1500.00} payment would finally close out my account and end things with Comenity. In fact, after I made that payment, in excitement, I kept checking my balance on Comenity and each time it said XXXX. That is exactly what it should be. XXXX balance. 
I receive a XX/XX/XXXX statement. I opened it expecting it to verify that Ive paid off my account. However, it says I owe {$31.00}, due on XX/XX/XXXX. The statement says this {$31.00} is for charged interest. There should be no interest charged when I paid off the remaining balance of what was owed on XX/XX/XXXX ( the date due for the last statement cycle ). In other words, no balance was carried over into the statement cycle. 

Issue : I effectively paid off my account balance in full on XX/XX/XXXX when I made that final payment of {$1500.00}. 
Given Comenitys latest statement, the XX/XX/XXXX statement, incorrectly claiming that I owe {$31.00} in interest, I emailed Comenity on XX/XX/XXXX and called them on XX/XX/XXXX, requesting/pleading with them to remove the {$31.00} charge because it was inaccurately placed on my account. 
o XXXX XX/XX/XXXX, I had to speak with two Comenity representatives. The first one was rude and failed to demonstrate a semblance of customer service or willingness to help. The second representative, a XXXX XXXX ( XXXX # XXXX ), explained that although my account was closed in XX/XX/XXXX but at some point in XXXX ( unknown exactly when ), Comenity Bank changed its terms and conditions of their credit card agreements. How or why these changes applied to my account when it was already closed ( they closed it in XX/XX/XXXX ) is my first issue. 
o XXXX XXXX further explained that these changes Comenity Bank made in XXXX somehow created a residual interest system. He tried to explain to me what that meant but it sounded like theyve instilled a practice to deceive and hide the interest from consumers like me. Again, there was no balance left when I paid off my account on XX/XX/XXXX with my {$1500.00} payment. There was nothing to charge interest on and yet, this {$31.00} interest charge shows up on the XXXX statement. With this residual interest floating after unclear timelines, theyve instilled a practice thats deceptive, inaccurate, and unacceptable. XXXX XXXX even subtlety inferred that many, even within Comenity, have issues with this residual interest practice. 
o I asked XXXX XXXX to remove the {$31.00} from my account but he refused because it is interest. He said if it were a fee, he could help but since it is interest, he cant remove it. 

My issue is twofold : Comenity is inaccurately charging me interest and I refuse to pay it and I respectfully ask for CFPB to intervene/investigate and assist ; and I also want it noted that the changes Comenity put in place in XXXX by implementing a residual interest plan shouldnt have ever even applied to an account that they themselves closed in XX/XX/XXXX. 

Please advise. What should I do?