Most likely you are testing in IE and have Display freindly error messages enabled.
Go to Internet Options -> Advanced and uncheck the "Display Freindly HTTP Error messages? option.
When I test you site using Fire Fox, I click the add to cart button on the 'http://jjsbs.com/WebTrendReports_OrderForm.asp", then I see the following error when "http://jjsbs.com/cart/cart_display.php" is loaded:
PHP Warning: require_once(../WA_eCart/eCartMembers_PHP.php) [function.require-once]: failed to open stream: No such file or directory in D:\Websites\www.jjsbs.com\cart\cart_display.php on line 3 PHP Fatal error: require_once() [function.require]: Failed opening required '../WA_eCart/eCartMembers_PHP.php' (include_path='.;C:\php5\pear') in D:\Websites\www.jjsbs.com\cart\cart_display.php on line 3
make sure the "eCartMembers_PHP.php" page is uploaded to the server.
It also seams your ASP Add to cart button is redirecting to your PHP cart page. You should change that to redirect to the ASP cart page.