Question regarding Apple's Online Store

On a recent order, the Apple Online Store used my SHIPPING address as my BILLING address. So when I contacted them about a problem and they asked for my BILLING address zip code, I gave it to them. They said that was wrong! I quickly figured it out (the two zip codes are the same except for the last digit) and gave them the SHIPPING zip code which was accepted.

After completing the primary business, the Apple rep reminded me to go into my Apple Store account and make sure the BILLING address is correct. However, I can't find a way to access my account without placing an order. Does anyone here know how to do this or if it is even possible?

BTW, I could not find a relevant discussion area so I just used Macintosh related device/topic.