WP eCommerce checkout page does not progress to PayPal

ShimmeringMesaGlass.com is the website I’m working on. The checkout process has been working smoothly for months. Suddenly this week, with no changes to the website, the checkout process gets stuck and doesn’t move on to Paypal. It just refreshes the checkout page. In an attempt to resolve the issue, I installed the newest version of eCommerce but it had no affect on the issue.