MailChimp for WooCommerce
MailChimp for WordPress works with Checkout for WooCommerce without any code required.
The only requirement is that you change the default WooCommerce hook that displays the subscribe checkbox. You can do this in the settings here:
We recommend this action:
That places the checkbox right above the complete order button:
If you would like to place it under the "Create an account" checkbox, you can use this action: