There's nothing unethical about this. The unethical part is the client who doesn't want to pay for the product they received according to the previously agreed

There's nothing unethical about this. The unethical part is the client who doesn't want to pay for the product they received according to the previously agreed terms. Now THAT is unethical, and those people should be shot, and their dog should be shot as well. Sending them a friendly repayment notice instead of doing what really needs to be done is a great act of kindness.