I am crossing my fingers, hoping that my upgrades work for you. I have successfully upgraded the COWOA mod for use with my 1.3.8 site and after much testing, I haven't found any errors. I have added the download to the wiki here:

COWOA Wiki

I have also upgraded my test site from 1.3.6 to 1.3.8 and I added the upgraded COWOA files to it if you want to test it:

COWOA test site

(The login page may be showing guest checkout twice. I don't have time to fix it at the moment but will get to it as soon as I can. The file is includes\templates\YOUR_TEMPLATE\templates\tpl_login_default.php)

Please let me know asap if you encounter any errors. Back up your files first please - I don't want to be blamed for anything! I tried my best!!!