Thanks so much for the replies. I have done the above with no different results.
(When I click continue, a lightbox begins to open... Open... Nope. CLOSE. I remain on the checkout.php page. I can get no further.
Please let me know as soon as you're able. Your help is very appreciated.
I noticed that the submitting form posts Security_Code_1, not Security_Code as in your manual code example, so I changed it... No change to results. I have attached the latest files. Please be so kind as to inspect.