I decided to email PayPal also to see if they had any ideas. They don't believe its an "international" issue but something else. Do you know if what they are referring to below is due to the plugin? Here was their response...
This is not a country issue but an issue with how the browser is interpreting the redirect. For example in IE 7 the redirect works but in FireFox 3.5 it does not and I get the PayPal homepage. This is because the form post url that is being used is not the full path and one browser does not allow PayPal to forward the information where the other does. For example this is the first line of the Add to Cart button on the page you referenced:
<form action="https://www.paypal.com" method="post" target="_self">
It should be directing to
<form action="https://www.paypal.com/cgi-bin/webscr" method="post" target="_self">
By replacing the domain with the full path url it will resolve the issue referenced.
Merchant Technical Services
PayPal, an eBay Company