I reported this issue some time ago, but there is still apparently no fix:
On the "Make a Payment" web page, the form for entering credit card information doesn't work. The "Expiration Month" and "Expiration Year" pop-up menus don't pop up.
I don't know if this is specific to the Safari web browser, but come on, millions of people use it! And a simple pop-up menu is not exactly the HTML equivalent of rocket science.
I've tried this on two different Macs running Mavericks and gotten the same result.