Alternatively - identify the order on your Manage All Orders page.  Click 'Create Postage Label' from the drop down list again.  At the top right of that page is a link to 'Manage Postage Labels'.  You might have to change the period at the top of the table - but you should be able to see the postage label you printed.  Under 'More Actions' there is a request to cancel the postage.  I generally use the reason that the postage label has been lost. 

Hope this helps -

Sarah