Credit One -I sent payment coupon with a letter and Credit One closed my account because of "unauthorized payment" - admitting that the payment coupon IS payment...They removed the credit line from my credit report and sent the account data to a "collections" company.  I sent Credit One a Conditional Acceptance, they ignored, I monetized CA with the Treasury who received it on 12/30/22 .  Still waiting.  The collection company sent me a letter with a coupon and option to mark "this is not my debt" which I marked and sent back and NEVER heard back from them!
3 medical bills paid (over $10K in total), and an old "tax" bill from when I used to file income taxes I paid via coupon plus a letter.  After I paid I did receive another "statement" but I ignored the second statement and filed IRS Form 3949a on the payment coupon I had sent and I sent a copy of the form to both the company AND the IRS and then I NEVER heard back.  This happened in Nov 2022.  I have no proof, but my word, because these were as I call them "one off" payments with no "online" statements to show proof.  I find "one off" payments to work more successfully than using the coupon on things that you could provide solid proof of payment- like credit card bills, internet etc... 
Capital One, I over paid, with the coupon and a letter.  They restricted my account and didn't credit the payment.  I did the CA process and monetized and sent to the treasury delivered on 12/30/22. Waiting for a remedy from the treasury. Last week, I decided to call Cap One and get some answers, because I'm curious....after 2.5 hours on the phone with them trying EVERY gaslighting technique they could muster, they said they "lost" the payment and would provide me with a "provisional credit" and "look into the matter" once I faxed them the proof of mailing and photocopy of the coupon I sent. They NEVER said the coupon WASN'T a form of payment and I was really clear about using the Payment Coupon.  I recorded the entire Phone call (a sample is posted in my free course)!!!  Sent CAPITAL ONE a fax of the proof they requested, confirmed the provisional credit (which was removed 3 days later) and am waiting for the written response they said they would send me.   I expect them to provide me with some gaslighting BS letter in which I will CA them AGAIN.
Friends, this is NOT going to be easy, they WILL NOT go down without a fight...but IMO, it's a fight worth fighting.
I'm convinced that the more people that learn about this and insist on their remedy...eventually the fraud will have to stop!