One thing that may be causing the problem, make sure that the code:
<?php
// WA eCart Redirect
if ($XOX_Shop->redirStr != "") {
header("Location: ".$XOX_Shop->redirStr);
}
?>
appears below the set session variable code. So when I said to move it directly above the doctype, I should have mentioned this code also has to be moved below both.
So it should be ordered:
Validation Code
Set Session Code
Location Redirect Code