I sent my coupon certified mail, to the CEO and headquarters of the Capital One. (I did receive my return receipt) I made sure to put all necessary information on the coupon as instructed by the How to Pay Bills with Coupon Course, I added my bene signature, along with UCC codes requirements. They sent me a letter closing my account and told me I still owe them money.
I have not tried to contact them about this yet, but realize that since they accepted my payment, I now have to contest the fact that they did not accept my attempt to settle my debt via certified mail.