try changing line 140 - 145 on the detail page:
<?php
// WA eCart Redirect
if ($WA_Store_Cart->redirStr != "") {
header("Location: ".$WA_Store_Cart->redirStr);
}
?>
to:
<?php
// WA eCart Redirect
if ($WA_Store_Cart->redirStr != "") {
session_commit();
header("Location: ".$WA_Store_Cart->redirStr);
}
?>