Feedback does not affect your seller account with eBay.
It is a public relations program that helps buyers decide on the trustworthiness of the seller.
You can add a Response to any feedback, your own or the customer's indefinitely.
"Hat as described. No request for return or refund." for example.
The most effective Responses are calm and factual.
Also, defamation over a $10 ballcap?
You have 3 other negative FB and one neutral over cancellations. Those Defects are going to have a lot more effect than any feedback.
Droppshipping is a slow death for online sellers. If you don't have it in your hands, don't sell it.