Yes, you can send your customers their tracking numbers once you've marked your orders fulfilled or when adding tracking numbers using EZ Fulfill. To do this, go to the Data Feed you're using and click the Send Fulfillment Receipt option in the Custom Configurations section.

Once you've clicked this option, save your Data Feed, and your customers will get an email with their tracking number on your subsequent fulfillment.

When using this option, your customers will also get emails if the tracking numbers are added after fulfillment or if you override existing tracking numbers on your order.

Fulfillment Receipt Template

When your order is fulfilled, or you add/update tracking numbers, we send Shopify a flag letting them know you want your customer to receive an email.

If you need to edit or change the email template, go to the notifications section of your Shopify admin and click the shipping confirmation and shipping update notification settings.

You can customize these notifications from your Shopify admin as well.  Shopify has excellent documentation on how to do this.

We do this so you can use the same email template using the Shopify admin or EZ Fulfill.  This also means the emails are sent from Shopify utilizing their domain authority and platform infrastructure.