Hi Adriana,
You can zero out the fields for the other 19 items, and just leave the amount of the one item you want to refund. Then press submit. We're working on new behavior in that page that won't mean you have to zero everything out in the future - we got that request from someone else as well.
You wont be charged transaction fees for that particular item once you refund- they'll be credited on your bill to you once you do this.