Yes, we've been doing excess postage refunds since last year.
Go to your orders page, and choose "send refund" from the drop down next to the order you want to refund (all or partial).
You are taken to a page on which you can select the reason for the reason, add an optional message to the buyer, and type in the amount of the refund.
You then click the "review refund" button at the bottom of the page, check all the details, and confirm.
