First, to enable Klaviyo integration, you need have the Parcelous Shopify app installed on your store and a Klaviyo account.

After that you have 2 ways to enable the integration:

1. Open the Parcelous app in your Shopify admin, go to the "Email Notifications" page and go to the "Klaviyo Connection" section. Click the "Connect to Klaviyo" button. This will open a new window where you can log in to your Klaviyo account and authorize the Parcelous app.

2. Go here: https://shopify.parcelous.com/api/klaviyo/external and follow the instructions.

After you connect your Klaviyo account, a new metric will become available to you: "Parcelous: New Tracking Events".

Once you enable the "Track Orders Automatically" option on the "Email Notifications" page in the "Settings" section, the Parcelous app will start automatically tracking your orders in background. If you also enable the "Send Email Notifications" option, the app will send an event to Klaviyo every time there is new tracking data available for an order under the "Parcelous: New Tracking Events" metric.

The events sent to Klaviyo contain the following data: the number of new tracking statuses compared to the last time the order was tracked, current status of the shipment and tracking links that lead to the tracking page at your store.

Here's more detailed information about the data being passed and how to use it in your email templates: