I think that it will be best to use a radio button here rather than a checkbox. Since it is using the onclick method to call the function there is not a good way to check for a second click on the checkbox. Otherwise you would need to submit the form and check the state of the box and that would defeate the purpose of the script. So, lets use the radio buttons instead...
Change
Code:
<input type="checkbox" onclick="flipdiv('checkoutbutton','show')"> I Agree to the Terms and Conditions
to be
Code:
No<input type="radio" name="agree" checked="checked" onclick="flipdiv('checkoutbutton','hide')" /> Yes<input type="radio" name="agree" onclick="flipdiv('checkoutbutton','show')">I Agree to the Terms and Conditions
I will update the original post again to reflect this...