BigCommerce Installation Guide
With BigCommerce, you want to install your account script just like you would your analytics script. In other words, just below your Google Analytics code (without touching or changing your GA code).
Here's how you install your Crazy Egg script on your BigCommerce site:
Click Advanced Settings.
Click website analytics.
Under the General settings tab (located on a tab at the top) make sure Google Analytics is checked.
Click the Google Analytics tab (located at the top next to General settings).
Paste your Crazy Egg script from your account just below your Google Analytics code.