First, if you have IE8, you can emulate IE7 by clicking the Compatibility View button just to the right of the address bar.

Second, I'm getting a security error message when I navigate to the link you provided, before I log in (looks like you're using a shared cert).

Third, I'm seeing a big green 'Place Order' button at the bottom of your one-page checkout in IE6, 7 and8.

Fourth, in IE7 and 8, I'm seeing Sorry, we are not shipping to your region at this time. Please contact us for alternate arrangements. in the shipping section (I used a Chicago address). That message doesn't appear in IE6, for some reason. I didn't actually check out in any browser.

Fifth - I've never seen a Zencart store that didn't have a Contact Us page. Could you explain your thinking there?