The Post office has a FAQ to address the problem:
Here is the response I got from their help system:
You can print a shipping label with Click-N-Ship <https://sss-web.usps.com/cns/landing.do> if you are using a Mac computer with OS 10.x or higher. Please note that the Safari browser will not work at this time due to a technical issue involving the use of Adobe’s advanced PDF elements and objects interaction with the Safari browser. Please use Firefox. If you follow the following steps, you should be able to set Firefox up before you print the label.
1) Open Firefox (If you do not have firefox, you can download it here:
http://www.mozilla.com)
2) Select the word Firefox located next to the apple icon to bring up the drop down menu
3) Select Preferences from the drop down menu
4) Select Applications under preferences
5) On the Applications menu, under Content Type, look for Portable Document Format or PDF, or LabelGenerationServlet
6) Across from that under Action, select to open with Adobe Reader
7) Once the change shows in the Application Menu, close the menu and your browser session
9) Start a new Firefox browser and sign into USPS.com and attempt to print the labels
Additionally, Apple Preview does not support printing the label, you must use Adobe Reader Version 7.0 or higher. If you require additional assistance, please contact us again
