Disallowed Key Characters

I’m getting an error on a form thats connected up to a payment gateway.

They send the values back to my “receipt page” via POST.

I’ve reduced the form down to one field for testing purposes, yet I’m still getting the “Disallowed Key Characters” issue.

I embed all the PHP code into one template and embed that into another template. Here’s all the code from the template I’m embedding:

<?php
$chargetotal = $_POST["chargetotal"];
?>
Total: <?php echo $chargetotal; ?>

Any help would be much appreciated!