The advice you've been given so far is totally incorrect. You cannot part refund. You must refund in full through an open return request - there is no other choice. And when you do, that will at least credit the selling fee back to your Ebay account. If you don't, the buyer will escalate the return request to Ebay, and Ebay will force the refund and slap a damaging defect on your account for not refunding voluntarily. And you won't get a selling fee credit. You agreed to abide by the Money Back Guarantee when you registered your account, so read it for yourself, so that you know what's expected of you, and what your responsibilities are. It's on every listing and in various other places across the site.
Now back to your apparent problem...
As this was buyer error, it is the buyer's responsibility to pay the return costs. The seller is only charged when a buyer claims that an item was received damaged, faulty or not-as-described. So either the buyer chose the wrong reason for the return from the list presented to them, or you actually set the listing up with free returns, or the charge you're seeing is not connected to the return request. So check out which of these it is, and someone will be happy to provide accurate advice.
@linjuliaa
