Unfortunately I do not have any credit card expenses to test Safari Enhancer, but on occasion I have found it necessary to re-select "MSIE 6.0" on a finicky webpage that requires IE....don't know if this will help. Although Firefox is my 2nd favorite browser behind Safari, you may have better luck with Camino. I have used it successfully on some military (.mil) websites that Safari and Firefox choked on.
You can download it here:
http://www.caminobrowser.org/
Another really good browser you might try is Opera.
http://www.opera.com/download/?platform=mac
At version 9 point something, it is a very mature and stable browser for the Mac.
FWIW, Safari seems to handle all the other drop down menus I've tried on FOCUS just fine.
Good luck and let me know if this works.
-JB