Yargh! I get so frustrated with the usps site. It is SO SLOW and took me forever to figure out how to print with the MAC.
Here's how, in case anyone else needs to know:
Don't bother with Safari, use Firefox. When it asks you how you want to open the file, save it to desktop. Then, click on the little icon so that you get a cursor. The name will say something like: com.usps.cns.web.pdf.JavaLabelApplet (not sure about that end bit) at any rate, delete everything after the ".pdf". Then it will want to know if you want to rename it as a pdf file, tell it yes. Then it will open in a window on your screen, and you can go up to file and tell it to print.
Annoying, huh? I'd almost rather go stand in line at the post office! I hope they get that sorted out soon.