Direct link to cart.php is not working

Hi

My direct link to /clients/cart.php is not working the first time you click however if you click again it works.

The link I am referring is the one in the green button called sign up which is placed in the top header to the right. Every other button sends me to the correct url which is /clients/cart.php.

First click sends me to clients/index.com instead of /clients/cart.php.
Second click sends me to correct url which is /clients/cart.php

My site is: http://www.snappyrack.com

Here is my code from the header.tpl: <li id="menu-item-20" class="client menu-item menu-item-type-custom menu-item-object-custom menu-item-20"><a href="http://www.snappyrack.com/clients/cart.php">Sign Up</a></li>

Any idea why it is not working on the first click?

Thanks
David