There are always posts about 'unfair' feedback and generally there are two sides to the story.
However, I do agree that feedback was just silly.
As stated, it is too late now for it to be removed. However, anybody who reads the feedback can clearly see that the buyer was in the wrong.
As you sell more items, your % will rise so, personally, I would not worry about it.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)