Nice idea... really wasn't ready to be released though. Here are some suggestions:

1) Don't write in your readme "all files are arranged in the correct Zen Cart v1.3x structure." when they aren't :) Among others, you need to:
- delete the empty /modules/pages/checkout folder
- the /includes/extra_functions folder needs to be /includes/functions/extra_functions
- the /includes/templates/tpl_checkout_default.php file should be /includes/templates/template_default/templates/tpl_checkout_default.php
2) while you are at it... delete the extra_functions file because its empty anyway - nothing but comments?? no code at all.
3) your /images folder is useless - just delete it
4) In your instructions... you suggest to change every reference of FILENAME_CHECKOUT_SHIPPING to FILENAME_CHECKOUT and then attempt to say what files that may affect... this is backwards. LEAVE ALL THOSE FILES ALONE!!! You already have the solution built-in (see below)
5) don't bother with your /includes/extra_datafiles/checkout.php file... just delete it. Instead, in the instructions have the user modify the includes/filenames.php file and change the line
Code:
define('FILENAME_CHECKOUT_SHIPPING', 'checkout_shipping');
to
Code:
define('FILENAME_CHECKOUT_SHIPPING', 'checkout');
. Your extra_datafiles file actually attempts to redefine a constant, which you can't do. This way is much better and eliminates any need to modify potentially dozens of files (78 on my site) in favor of just editing one.

That's as far as I got. I did NOT install this mod... once I saw the hard-coded no-no's I gave up.

Again, good idea! But remember that unrefined mods probably end up causing people more harm than good... release it as a BETA and get it tested first!

Good luck and keep trying!

- Steven