Eunice here from the Shopify Social Care team. Thanks for posting and welcome to the Community!
Sending tracking numbers to your customer shouldn't cost you a thing, but you would need to purchase a shipping label with tracking in order for the tracking number to be populated. How you get hold of this tracking number depends on whether you're fulfilling the order yourself or using a third-party fulfillment service, such as the dropshipping app Oberlo, or a print-on-demand app such as Printful.
Fulfilling the order yourself
If you keep stock on-site and you pack the order yourself, then you'll get the tracking number when you ship the package at the post office or book a courier to pick it up.
You can add the tracking number when manually fulfilling an order in your Shopify admin. The number will be included in the shipping confirmation email that is sent to the customer
You can add a tracking number at a later date too, once the order has been fulfilled. Instead of receiving a second shipping email confirmation, the customer will be sent a shipping notification email containing the tracking number:
It's important to note that if you purchased a shipping label with a tracking number through Shopify Shipping, the tracking number will be added to the order in your Shopify admin automatically.
Fulfilling via a third-party
If you don't keep the stock on site and you don't physically pack the order then you won't have the tracking number on hand to give to the customer in the same way as above.
Most Shopify apps are built by third-party developers, not by Shopify. If you need help with an app or sales channel that was built by a third-party developer, you'll need to contact the developer directly.
Hope that helps! If you have any questions just sing out here.