I do not think she has gone back and had a look at the ad. She thinks you just picked the size out to send to her without asking her. I have a feeling she is hoping for an exchange or else for you to send replacements. I am not sure.

So I think you need to reply.

Just say something like you are sorry she is not happy but if she has another look at the ad she will see that it was clearly and prominently stated in the listing that the socks were size 3-9 . And that you do not carry size 10. 

You did nothing wrong. It is her mistake. I see you allow returns within 30 days. You could also say in your reply that  you accept change of mind returns and she is able to return the socks for a refund as long as they have not been opened and are still in original condition, but she will need to pay the return postage.