I have looked at the USPS online Click and Ship offering. https://sss-web.usps.com/cns/landing.do
It seems like the requirements are quite simple. You need a modern browser like Firefox 3.x, Internet Explorer 6.x, 7.x, or Safari. You also need a printer and a PDF reader like Adobe Reader.
I have tried the sample labels on Mac OS X and on Ubuntu Linux. I didn't need to install anything on either system to get things to work. I didn't see a downloadable Mac program, so we may not be talking about the same thing. The online service seems to be very straight forward and easy enough to do, at least for low postage volumes (10 to 20 packages per day).
Um, I run Firefox and Safari, but under System Requirements it says "Browser - Microsoft Internet Explorer 5.0 or higher, or Netscape Navigator 6.0 or higher. Hmmmm.