If you'd like to know which of your keywords best leads to sales, you should integrate your Google AdWords account with your store.

1. Go to your Google AdWords back-end and open the "Tools → Conversions" page.

On the conversion actions page click on the “+ CONVERSION” button

Next click on the “Select” button under the “Website” bucket

Now you need to set up your conversion.

a) Enter the name of the conversion you want to track, click “Done”

b) You are now can set the parameters for your conversion tracking. In the “Value” section choose the second option, and enter the conversion value. When you are finished with this click “Done”

c) In the next section choose the way to count your conversions. Choose between “Every” or “One,” and then click “Done”

d) Next choose the Conversion window you want to track, once completed, click “Done”

e) Choose what Category of conversion you wish to track. Select “Purchase/Sale”, click “Done”

f) The Include in "Conversions" window allows you to control how you bid for conversions and to customise the data in your reports. Choose “on” and click "Done"

g) Choose the Attribution model. Attribution models let you choose how much credit each click gets for your conversions. More details: About attribution models. Click "Done".

Then, click the “Save and continue” button.

On the Settings page, scroll down and you will see your code inside of a text box:

You need to add this code to the “Thank you for your order” page in your Ecwid Control panel. Copy your own tracking code (not the one above) to your clipboard.

Only sales that were generated by the clicking of your ad will be recorded.

Unless your “Thank you for your order” page is visited after a transaction, the conversion tracking code won’t run. There will be no tracking if your customer is not returning to Ecwid after a transaction has been completed using an offsite payment gateway. 

a) Log in to your Ecwid control panel and make sure that you are on a paid plan which allows you to embed tracking codes to the last checkout page. Open the "Settings → General → Cart" page and find the "Checkout options" section. Enable the "Tracking code on "Thank you for your order" page" option.

b) Then enter your saved tracking code and save the changes.
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A detailed instruction is also available at support.google.com/adwords/bin/answer.py?hl=en&answer=1722054

If you inspect the page source code via the browser's 'View page source' tool, you won't see the tracking code you entered in your control panel when setting up Adwords tracking. That is actually expected behavior. Ecwid is an AJAX application and everything in your store is delivered to the web page dynamically by JavaScript. The tracking code is loaded dynamically as well - when the order confirmation page opens, the tracking code loads and executes in background. It is how it is supposed to work. So please do not worry you do not see it in the page source. If you want to inspect the code and debug it, you should use developers tools in a browser (e.g. the built-in Chrome's developer tools or FireBug in FireFox).