USPS.com has a feature that allows one to print shipping labels but the printing process is (by default) fairly Windows-centric: the file passed to the computer when you click “Print Postage” ends in an oddball filetype OS X doesn’t know anything about (the file extension is “LabelGenerationServlet.”)
Safari doesn’t do squat when you click the link and Firefox doesn’t appear to (really it downloads the file but since OS X is stymied by the file it just sits there after download.)
Anyway accessing USPS.com from Google Chrome for OS X works ’cause Chrome auto-converts the downloaded file to PDF upon download.
Take that USPS.com!