Thanks for getting back to me with that. I was able to visit the ticket and check out the site and test this out. From what I can tell there are a couple of things going on with the checkout page.
The first issue is that you have an add to cart from form on this page and the trigger is set to any form post. Do you need this on the page? Part of the problem with this is that at the top of this server behavior is a line that is clearing the cart. So any time that you have any info posting to this page the cart will be emptied.
The second issue that I think might be present is that your server behavior for sending the user to PayPal does not seem to be triggering properly. You can check on this by editing your Authentication for PayPal Express Checkout server behavior that is on the page. In here try updating the trigger to any form post. After you make these updates test the page to see if it is working as you expect it to. If there is a problem with the trigger for this server behavior this update should resolve the problem.