Shipping Estimate error - zip code not required?

If I click the "estimate shipping & taxes", then enter:

Country: United States
State: Arizona (or anything else, doesn't matter which one I select)

the "post code" field is not required. So, if I hit the "get quotes" button, I get the dreaded error:

111210: The requested service is unavailable between the selected locations.

So, how do we make the "post code" field required? If they dont' enter a zip code, the 111210 error doesn't tell them what the problem is - but we can't change that error since it comes from UPS.

Help?

Bumping this topic. Using the latest version of OpenCart and I'd like to know the same thing.

You can make postcodes required in System > Localisation > Countries. Edit the "United States" country and make the postcode required.

I found that FedEx requires post codes for these countries:
http://www.fedex.com/us/developer/produ ... rmats.html

One missing from the list is Hungary.