I thought I was editiing the wrong file, but was just going with the OP's opening instructions:

Quote Originally Posted by numinix View Post
[URL="http://www.numinix.com/index.php?main_page=document_general_info&cPath=160&products_id=760"]

TIPS:
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Fast and Easy Checkout for Zen Cart is activated by loading an extra_configures file called checkout.php before the filenames.php file.
This causes definitions for the checkout links to be defined to direct customers to the new integrated checkout page. To deactivate the script without
running the uninstall.sql and deleting all of the files, simply rename or delete the file includes/extra_configures/checkout.php.
To personalize the checkout, you can create your own CSS file and then edit the classes found within
includes/templates/YOUR_TEMPLATE/templates/tpl_checkout_default.php
This file contains the layout of the page and can be re-organized by moving the fieldset blocks to however you want.
You may also want to redefine the TEXT and ERROR definitions found in checkout.php. Do NOT redefined the FILENAME definitions or the script will not work.
I had initially looked at tpl_checkout_stacked.php but that's some scary looking code.... will look again now :-)

I don't have that problem. My checkout button works a treat, always has. Sounds like you've somehow corrupted a file. Try uploading the original files that shipped with the mod once more, but watch for the MERGE on the shopping_cart_default.php file.... hopefully you didn't overwrite an existing file.