Log into your Paypal account and find the payment on the summary page. Click on it and you'll see the "Refund this payment" link. Click on that and on the next page change the amount to what you want to refund, click on Continue and then submit.
That's for postage, etc.. If the buyer has opened a "Not as described" case on ebay there's an option to issue the partial refund without return from there.
