Have the provide a receipt from USPS for the supposed COD.
If they can provide a receipt, then refund them. You made an error and they had to pay. They might be annoying, but their complaint is legit.
If they cannot provide a receipt, then they might trying to scam you. You need to then look and see if they are someone you want to keep as a customer. If you don't mind losing their business from now on, then tell them to try to scam someone else, and that you fulfilled your end of the contract. If you want to keep them, then bite the bullet and refund them.
no subject
If they can provide a receipt, then refund them. You made an error and they had to pay. They might be annoying, but their complaint is legit.
If they cannot provide a receipt, then they might trying to scam you. You need to then look and see if they are someone you want to keep as a customer. If you don't mind losing their business from now on, then tell them to try to scam someone else, and that you fulfilled your end of the contract. If you want to keep them, then bite the bullet and refund them.