this is a case of buyer changed thier mind, as you did everything posssible to provide her with the sizing information, so you donot really want to do the refund under the Paypal refund as this could be seen as a defect on your part, and as you only want to refund the cost of the item and not the postage, would copy her Paypal email address from her payment details and then via the Pay or Send button at the top of your Paypal account, send her the item cost only via her email address.