@cambreco yes, that's right. As you say, it's Step 1 (search), Step 2 (select transaction from amongst items list), then go back up to the Step 1 area and edit the Amount and add a comment, then Submit button which finalizes the partial refund.

I did have initial trouble getting to edit the Amount/Comment but it did resolve and I've sent 5 partial refunds this week successfully.

PS - one other trivia I learned: the transaction needs to have settled fully (clear the bank) to be eligible for partial refunds. If an item is Pending status still (you just took the payment that same day, for example) one cannot partial refund it but it would be Voided instead.