I have just used Fast and Easy Checkout for Zen Cart at http://www.zen-cart.com/downloads.php?do=file&id=594. It says that it is compatible with zencart 1.5 but it is not. Also it is not structure similar to zencart 1.5 structure. I normally use Winmerge to modify files while installing zencart modules but for this i am not able to use it because there are complete files are different.
Anyway i have modified files very carefully but i am getting now error in the checkout page which normally not comes in zencart 1.5. In Zencart 1.5 when i click on "Checkout" at that time redirects to mainpage='checkout_shipping' but now it redirects to main_page='checkout' and also it shows blank page after header and i have check log error in cache folder and it says
"[27-Oct-2012 09:01:13] PHP Warning: require(includes/templates/template_default/templates/tpl_checkout_default.php) [function.require]: failed to open stream: No such file or directory in K:\kishan\XAMPP-New\xampp\htdocs\test\talisman\includes\templates\jewellery\common\tpl_main_page.php on line 87
[27-Oct-2012 09:01:13] PHP Fatal error: require() [function.require]: Failed opening required 'includes/templates/template_default/templates/tpl_checkout_default.php' (include_path='.;K:\kishan\XAMPP-New\xampp\php\PEAR') in K:\kishan\XAMPP-New\xampp\htdocs\test\talisman\includes\templates\jewellery\common\tpl_main_page.php on line 87".
Please let me know if somebody have used this plugin and know how to figure it out or give me any other free or premium plugin to make one page checkout by using AJAX.
The error message is telling you that neither the template_default nor your custom template folder contain the required tpl_checkout_default.php file ... which would be included in the plugin you're attempting to use.
So, the first step in the solution is to upload that file to your custom "jewellery" template folder.
Additionally, if what you're really asking for is an entirely ajax-driven version of FEC, you'll need to purchase that from the Numinix website, as that's where they sell and support it.